DE3329578C2 - - Google Patents

Description

The present patent represents an additional patent to patent 33 27 638. It describes a process for the production of molded articles having pores by extruding a homogeneous, single-phase, liquid mixture of one or more polymers and one or more at the temperature of the production of the Mixture of liquid mixture partners, the mixture having a range of complete miscibility and a miscibility gap in the liquid state above room temperature and having a solidification range above room temperature, into a cooling device containing cooling liquid and stripping off the formed body, the polymer / mixture partner mixture being at a temperature above the miscibility gap with an average linear velocity v ₁ through a nozzle from top to bottom in a cooling liquid which does not or only insignificantly dissolves the polymer at cooling temperature and which has a temperature below the solidification point nktes, and the extruded mixture is led from the point of entry into the cooling liquid at least to the point where it begins to solidify 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 at least 20% holds smaller than v ₁, the molded article after the onset of solidification of the polymer is deflected and passed from the bottom upwards through a second zone, withdrawing from the cooling liquid, and the level of the cooling liquid both at the point of entry of the mixture into the cooling liquid and the level of Cooling liquid at the point of exit of the molded body from the cooling liquid keeps constant.

It has now been found that following this procedure advantageously also molded bodies having pores made from polyvinylidene fluoride (PVDF), the are well suited for a number of applications.

Polyvinylidene fluoride (PVDF) is because of its chemical Nature an interesting material. Especially his low chemical reactivity as well as its hydrophobic Properties make it a valuable raw material for the most diverse areas of application. As an an example weatherproof paints and coatings are mentioned here. It therefore made sense to try to find pore-containing PVDF Moldings, e.g. B. in the form of membranes. These membranes were expected to be suitable for the have a wide variety of separation processes used.  

In particular, they should be used with advantages there where other polymers, due to their chemical nature, d. H. their instability against some organic substances, their hydrolysis sensitivity in acidic or alkaline Medium or its susceptibility to oxidation, not or only can be used conditionally.

Therefore, they are not just processes for the production of membranes known from polymers, but also PVDF membranes different embodiments. These are made according to procedures manufactured in the same or slightly modified Form also for the production of membranes from other polymers can serve. So describes the US-PS 42 38 571 Process for producing a porous material, e.g. B. made of PVDF, starting from a solution of the polymer the solvent is evaporated to a precursor to obtain porous polymer, which then undergoes a subsequent reaction is subjected. The European application document 40 670 specifies a process for making porous PVDF, in which from a solution of the polymer in a special Mixture is produced by a membrane that the Pour solution onto a solid surface and then in contact with a coagulating medium. The European Application document 37 836 describes porous hollow threads made of PVDF, which are made by using a solution of the polymer extruded and then coagulated using a coagulating solution.

In addition to these processes for the production of porous PVDF Membranes are also special processes for the production of  Membranes or porous structures made from other polymers known. DE-OS 27 37 745 describes a method for the production of microporous polymer structures in which a homogeneous solution is assumed, which when cooling first goes through a liquid two-phase system before Solidification takes place. This is compared to others Process advantages in the design of the pore system achieved.

DE-OS 28 33 493 describes a process for the production porous hollow fibers, which allows the pore system to influence specifically. Again, segregation a polymer solution in a liquid two-phase system Used before solidification in a cooling bath entry. Finally, DE-OS 30 26 718 describes Process for the production of porous polymer hollow fibers, at which is a homogeneous polymer solution when cooled in one Bath first forms two liquid phases and the solidification in the bathroom under special conditions.

The described processes for producing porous Polymer structures such as B. membranes have next to some However, advantages also, e.g. T. serious, disadvantages. So it is in those cases where precipitation or Solidification of the polymer takes place without a liquid Two-phase system is run through, not or only conditionally possible, the pore system in terms of pore size and characteristics to influence specifically.

Manufacturing processes in which a separation of the Polymer solution in two liquid phases before solidification  takes place, but at which the solidification temperature or below room temperature are disadvantageous afflicted to form the porous polymer structure a washing or extraction process is necessary. Here is the rate of pore formation depends on the rate of diffusion of the extractant through the gradually solidifying outer layers inside. The process speed or the thickness of the As a result, the membrane to be produced is subject to strong Restrictions. In addition, the dimensional stability during the manufacture of the products, especially in the case of Hollow threads, often to be desired. The manufacturing process, in which go through a liquid two-phase system and where the polymer solidifies above room temperature takes place, however, have clear advantages on. They are for the manufacture of porous structures from different polymers well suited and lead to good dimensional stability of the products. It has however, it was found that even with this method Certain difficulties can arise when porous moldings be made from polymer compositions should be compared to mechanical during processing Stress are sensitive. This vulnerability to mechanical stress is particularly important given low-viscosity polymer mixtures.

Even after the cheap process, where a mixture a homogeneous liquid polymer composition Cooling first passes through a liquid two-phase area and then solidified above room temperature, it is  difficult to form these mixtures into porous molded articles to process when the viscosity of the processed Mix before extrusion through a nozzle below a certain range, namely below approximately 15 Pa s lies. Especially in the case of PVDF compositions however, the viscosities are often lower than in the case liquid mixtures containing other polymers. About that In addition, it is often desired to have a relatively low viscosity Process mixtures to create special designs of the pore system.

The task on which the invention of the main patent was based, was, therefore, an improved method of making Molded bodies, in particular membranes in Form of hollow threads, tubes or foils deliver. The process should have good dimensional stability Ensure products during their manufacture and therefore the processing of even low-viscosity mixtures without Difficulties with products more constant and targeted allow variable properties. One of the main requirements the process to be developed was therefore the mechanical stress on the polymer composition at least until the onset of solidification to keep the polymer low because of the composition a certain stability only after the onset of solidification wins against mechanical stress.

According to the main patent, this object was achieved by a process for the production of moldings having pores by extrusion of a homogeneous, single-phase, liquid mixture of one or more polymers and one or more mixture partners which were liquid at the temperature of the preparation of the mixture, the mixture above room temperature in liquid state has a range of complete miscibility and a mixture gap and has a solidification range above room temperature, into a cooling device containing cooling liquid and stripping off the formed body, wherein the polymer / mixture partner mixture at a temperature above the mixture gap with an average linear velocity v 1 conveys a nozzle from top to bottom into a cooling liquid which does not or only insignificantly dissolves the polymer at the cooling temperature and which has a temperature below the solidification point, and the extruded M ischung leads from the point of entry into the cooling liquid at least to the point of the beginning 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 at least 20% smaller than v ₁, the After the solidification of the polymer has begun, the molded article is deflected and passed from bottom to top through a second zone from which cooling liquid is drawn off, and the level of the cooling liquid at both the entry point of the mixture into the cooling liquid and the level of the cooling liquid at the exit point of the molded article the cooling liquid keeps constant.

According to this method, as has now been found, it is also possible to produce molded articles made of polyvinylidene fluoride (PVDF) which, because of its chemical resistance, is an interesting material if the speed v ₂ is kept at least 25% lower than v ₁.

Through the process steps mentioned, pores succeed having molded body made of PVDF, z. B. membranes in the form of Produce hollow threads, hoses or foils that are characterized by good dimensional stability. It can save Membranes but also porous PVDF threads after the process getting produced. The process steps succeed as explained in more detail below, the mechanical load, which the PVDF mix up to the time of the beginning Solidification, i.e. beginning to retain its shape is to keep lower than with known methods. Thereby it is also possible to use relatively low viscosity Process mixtures to form PVDF, what after previously known methods not possible or only possible to a limited extent is. Following the procedure detailed below products can be more consistent and reproducible Generate quality while in manufacturing according to date known methods, especially in the case of processing low viscosity polymer mixtures, uncontrollable Fluctuations in quality occur, which are often too great mechanical stress on those that have not yet solidified or form-stabilized polymer mixtures to let. The procedure described in detail below allows the pore size and characteristics to be determined Variation of process parameters in a wide range reproducible and targeted.

The PVDF molded articles for the production of the pores The mixture used is made up of PVDF and one or more Mixing partner (s) inert to PVDF. Inert here is understood to mean that the mixing partners do not chemically change the PVDF. At least one of the Mixing partner must be at the temperature of manufacture  Mixture should be a solvent for PVDF so that a single-phase liquid mixture is obtained. The or the Mixing partners must also have according to type and quantity be chosen so that the mixture cools down first passes through a temperature range as a result of segregation, in which two liquid phases occur side by side and only afterwards with the formation of a solid molded body stiffens. The one of the two formed after segregation Phases issues a liquid phase depleted in PVDF Mix partner (s), the other one to mix partner (s) impoverished liquid enriched with PVDF Phase. The latter leads to PVDF on further cooling Molded body. Both the temperature at which segregation occurs as well as the solidification temperature must be above Room temperature, to ensure that without additional steps such as cooling to below room temperature or extraction of the solvent on the shaped body on cooling Room temperature is maintained. The solidification temperature is preferably above 50 ° C. It is possible in addition to other solvents in addition to a solvent such as non-solvents, pigments, thickeners and surfactants to be used, provided that the above Conditions met will. Especially the addition of a non-solvent for PVDF usually brings advantages. The addition of a non-solvent causes no substantial change in the solidification temperature, but increases, depending on the type and quantity of the non-solvent, the separation temperature. This will the temperature range increases in which two liquid Phases exist side by side and thus the scope for a targeted variation of the pore system. Besides leads the targeted addition of non-solvent in selected  The type and amount that more leeway is gained, as to the type and amount of the solvent. The selection some solvents either by type or quantity is excluded without adding non-solvent because that mixtures that only use these solvents included, not to form liquid two-phase systems are qualified.

As mentioned above, the process also allows low-viscosity Process mixtures. This possibility is specially designed for the production of pores Moldings made of PVDF are important because they are single-phase liquid mixtures containing PVDF, often lower Viscosities at the temperature of the promotion by the Have nozzle than solutions of other polymers. The through the method given, compared to known methods leads to greater latitude in terms of viscosity greater scope in the composition of the mixture, what PVDF concentration, type and amount of solvent and possibly other mixing partners. This increased In turn, scope leads to more opportunities 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 values accept down to 2 Pa · s. The is preferably Viscosity between 5 and 35 Pa · s. This leads to the proportion by weight of PVDF in the mixture is about 10-90% and the weight percentage of solvent is about 90-10% can be. The PVDF content is preferably however between about 15 and 35% by weight.  

The PVDF used to make the mixture can be of commercial quality. The molecular weight However, it must be at least so high that the polymer is capable of thread formation.

Are as solvents for the preparation of the mixture in principle all substances suitable that are a mixture result, which the requirements set in claim 1 Fulfills. However, room temperature is particularly preferred liquid substances of low toxicity, the a boiling point well above the segregation temperature the PVDF mixture produced with them have. Glycerol triacetate, Glycerol diacetate, 2- (2-butoxy-ethoxy) ethyl acetate or mixtures thereof. The glycerol diacetate can the 1,2 or the 1,3 isomer or a mixture of both be. In the event that with the addition of a non-solvent can also be used as a non-solver Substance or any mixture of substances be selected, provided the requirements set out in claim 1 are fulfilled. To be particularly advantageous especially in connection with the mentioned preferred Solvents, have di-n-octyl adipate and castor oil or mixtures thereof.

The homogeneous liquid mixture is used for further processing conveyed through a nozzle by known methods and then enters a cooler that contains a coolant contains. The nozzle is depending on the one you want  Final product, e.g. B. as a hollow thread nozzle, nozzle for production formed by hoses or slot nozzle. To the mechanical Strain on the mixture before the time of Keeping the beginning of solidification low can be particularly difficult be advantageous in the case of the production of hollow threads, the inner lumen through metering is not, as usual, one Gases, but to form an internal filling liquid. The internal filling liquid can be added in the nozzle take place or at the point where the mixture consists of the nozzle emerges. A liquid must be used as the internal liquid be chosen, the PVDF at the temperature of the Delivery through the nozzle does not solve. Preferred examples are glycerin or a liquid that is used in the manufacture the PVDF mixture was used. The advantage working with an internal filling liquid instead of Frequently used gases consist of a liquid in the subsequent cooling because of its lower Thermal expansion coefficient for lower volume work and thus to increased dimensional stability of the PVDF molded body leads during its formation. In addition, the use offers a liquid instead of a gas has the advantage that the specific weight of the mixture leaving the nozzle can be set specifically in broader areas. Thereby it becomes possible to change the speed during the following almost free fall of the mixture or the speed from the entry of the mixture into the cooling liquid to the point where solidification begins vary. In the event that the inner lumen through a gas nitrogen is preferred.  

The PVDF mixture emerging from the nozzle enters a cooling device which is filled with a cooling liquid. There may be an air gap between the nozzle and the point where the mixture enters the cooling liquid. This makes it easier to achieve the required temperature constancy at the nozzle than when the nozzle is in contact with the (colder) cooling liquid. Since it is essential for the process to keep the mechanical stress on the mixture as low as possible at least until the start of solidification, the nozzle is mounted vertically or almost vertically above the point of entry of the mixture into the cooling liquid, so that the mixture emerges from the In almost free fall, the nozzle enters the cooling liquid. The mixture is cooled and solidified in the cooling device to form the shaped body. For this purpose, the device is filled with a cooling liquid. Cooling liquid can be metered in continuously during the process, and the direction in which it flows through the device can be the same or opposite to the direction of movement of the PVDF mixture or of the shaped body formed. In this case of continuous metering, the metered 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 entry point of the mixture into the cooling liquid. In this case, there is an overflow device at the exit point of the formed body, at which the cooling liquid leaves the device. In the case of the opposite direction of movement, the entry and exit points of the cooling liquid are swapped accordingly. However, it is also possible to work under steady-state conditions, ie there is no continuous metering of cooling liquid, but only the losses which result from the fact that the formed body takes cooling liquid with it. Here too, of course, the cooling liquid in the device must have a temperature below the solidification temperature of the mixture. In this case, in order to maintain constant temperature conditions in the device, it is expedient to carry out external thermostatting of the device. Depending on the process variant, the necessary maintenance of constant temperature conditions takes place either through the flowing cooling liquid or through external thermostatting or both together or through a temperature equilibrium which is established during the process. In the case of large dimensions of the device, external thermostatting is recommended. In this context, constant temperature conditions are not understood to mean that the temperature of the cooling liquid has the same value at every point in the device - which is not possible at all due to the supply of the polymer mixture at a higher temperature - but that the temperature prevailing there is at every point of the device Temperature does not change or changes only slightly during the process. However, there may be a temperature gradient across the length of the device. It is essential for the invention that the polymer mixture is subjected to as little mechanical stress as possible at least until solidification begins, ie until shape stabilization begins. It is therefore necessary to ensure that the mixture does not accelerate due to the cooling liquid - this applies if the mixture and cooling liquid move in the same direction - and does not slow down too much - this applies to the opposite direction of movement. Therefore, the mean linear velocity v ₂, measured in the direction of movement of the polymer mixture with which the cooling liquid flows through the device, in the zone between the point of entry of the mixture and its beginning solidification must be at least 25% lower than the speed v ₁ at which the Mixture emerges from the nozzle. In the event that the cooling liquid flows through the device in the direction opposite to the direction of movement of the mixture, this value of at least 25% is of course always satisfied, since the speed of the cooling liquid has a negative sign in this case. This provides a wide margin for the speed of the cooling liquid. For the same direction of movement, this is limited on the one hand by the limit of 75% of the speed of the polymer mixture given above. In the case of the opposite direction of movement, this speed may of course not have arbitrarily high values, but finds its limit where the relative speed between the polymer mixture and the cooling liquid is too high. This limit value depends on the respective process parameters and is easy to determine with just a few experiments. The speed in the case of the opposite direction of movement finds its limit where deformation or tearing of the not yet stabilized polymer mixture occurs. A rule of thumb for the limit of the speed of the cooling liquid in the case of the opposite direction of movement is a value of approximately

where v ₁ means the speed of the polymer mixture in m / min at the nozzle outlet. If the average speed of the cooling liquid were to be set to the same value that the mixture had when it emerged from the nozzle, the cooling liquid in the immediate vicinity of the mixture would have a higher value than the above-mentioned nozzle outlet speed because of the speed distribution in flowing liquids and would accelerate the mixture .

The speed of the cooling liquid, which is crucial Significance in the zone between the entry point of the Polymer mixture into the cooling liquid and the starting point Solidification of the mixture is controlled will. This is made possible by adjusting the level of Cooling liquid both at the entry point of the polymer mixture into the cooling liquid as well as at the exit point keeps constant. For this reason, is located at the Exit point an overflow device. In addition to the apparatus Measure is done to control the speed by adding cooling liquid accordingly. in the Case of a stationary bath, d. H. in the case where the speed the cooling liquid has a value of zero Dosing of course only compensates for losses.

In addition to the possibility of a special advantage of the method, 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 A wide margin is given as to what viscosity and Temperature of the cooling liquid and thus the type of cooling liquid concerns. In known methods, this is Scope much more limited because of the associated mechanical stress of the not yet stabilized Polymer blend.

Normally, the level of the cooling liquid on the Entry and exit not only constant held, but the level is in both places too same high. However, there can also be a height difference between both are present.  

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

• 1. hollow thread spinning nozzle,
• 2. coolant metering and tempering attachment,
• 3. stabilization zone of the hollow thread,
• 4. cleaning drain for coolant,
• 5. Overflow cup with level control device for Coolant,
• 6. Degree of deduction,
• 7. coolant supply from the thermostat,
• 8. Coolant drain to the thermostat.

Description of Fig. 1

To be a good observation during the process to create, the U-tube is largely made of glass.

The tube, essentially composed of five parts has a diameter of approx. 4 cm with a leg length of approx. 1 m.

a) Coolant metering and tempering attachment

To as quickly as possible from the feeding of the PVDF mixture Compensate for the temperature change caused is in this embodiment, the entry point for the cooling liquid near the entry point of the hollow thread.  The temperature jacket supports this function. At the same time this outer jacket and the metering openings ensure at the top of the pipe for a distribution of the Liquid flows and allow one if possible gentle treatment of the still unstable hollow thread. The approx. 5 cm² surface entry opening of the cooling tube ensures on the one hand sufficient scope for thin and thick hollow filaments or hoses and on the other hand reduces one greater surface unrest in the cooling bath.

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

In this area, the hollow thread runs through during cooling the stage of segregation into two liquid Phases and solidification. Depending on the recipe, nozzle temperature or cooling conditions can be the beginning Solidification or stabilization in the first or second third of the tube can be observed. Man recognizes this from the fact that the initially transparent Thread becomes increasingly milky and starts from the spot Stiffens to a certain final turbidity.

c) deflection of the thread

The stabilized thread can be deflected without that it gets deformed. In contrast to other embodiments the cooling device is not here Deflection wheel or no roller installed. The after the 1st zone downward pipe extension allowed convenient handling when piecing. The sinking Hollow thread collects here and can be used Wire pulled out of the outlet leg (2nd zone) and be placed on the trigger wheel.  

d) outlet leg

Here the thread reaches the desired final temperature.

e) outlet cup with overflow device

This device is made of metal. The height adjustable The middle section determines the overflow height of the Cooling medium and thus the normally level Height of the cooling liquid under the spinneret. An air gap can thus be created in a simple manner between the nozzle and the cooling liquid.

Components of the device shown in Fig. 2:

• 1st nozzle,
• 2. Cooling liquid metering with cooling pipe, inlet cup and overflow drip pan,
• 3. cooling pipe (transparent),
• 4. deflection roller,
• 5. cool box with sight glass,
• 6. Overflow device with level control
  (adjustable: level with the inlet or slight level difference),
• 7. Trigger wheel.

It is essential for the invention that the PVDF mixture between the time it leaves the nozzle and the As little as possible when the solidification begins is mechanically stressed, be it by pulling or by Shear forces. In particular, the mixture in this area not mechanically strong due to the cooling liquid  are influenced, such as. B. by acceleration. In addition to the Above mentioned control of the speed of the cooling liquid this is achieved by mixing the mixture in a first zone from the entry point of the mixture into the cooling liquid until solidification begins is enough, is led from top to bottom. These Zone represents a relatively narrow channel surrounded by walls represents, e.g. B. in the form of a cylindrical tube whose diameter is significantly lower than its length. The spot beginning solidification of the mixture up to which the mechanical Stress must be kept as low as possible, can be determined easily. This happens through Observing the changes that the mix is after experiences their exit from the nozzle. First takes place as a result Cooling the segregation into two liquid phases instead. The formation of two phases leads to an increase in viscosity ahead. A cloudiness begins with the development of two phases the mixture that persists until solidification begins reinforced. Because the solidification on the outer layers begins and then progresses inward, takes the visually perceptible cloudiness only between the time segregation until solidification begins and then doesn't change anymore. The place starting Solidification is the point where there is no increase the cloudiness is more noticeable. It is easy to do and pretty much determine.

Only after the point of solidification begins does the formed body have a certain stability which permits somewhat greater mechanical stress. Therefore, the deflection device may only be located after this point; however, it is not necessary that it be placed immediately after this point. After the deflection, the partially or completely formed solid molded body passes through a second zone of the cooling liquid, in which it is guided from bottom to top. Redirection and guiding from bottom to top are necessary in order to be able to use a device in which both the entry point and the exit point of the cooling liquid can be kept at a constant liquid level. This second zone, in which the molded body is guided from bottom to top, does not have to be channel-shaped like the first zone, but can be a zone with wider dimensions, e.g. B. in the form of a tub, as shown in Fig. 2. However, it must not coincide with the first zone, ie the molded body must not be guided from the bottom to the top in the channel-shaped first zone between the point of entry and the point where solidification begins. However, the second zone can also be designed as a channel-shaped zone, as z. B. is the case when a U-shaped tube, as shown in Fig. 1, is used as a device.

It is particularly advantageous if the cooling liquid at the point where the PVDF mixture enters it, a slight, d. H. up to 20% lower specific Has weight than the mixture. So prevented the mixture sags too quickly or too quickly strong braking due to large differences in density. This slight difference in specific weight can be in addition to the choice of a specific cooling liquid also achieved by having the specific weight of the  Polymer mixture changed, e.g. B. by the use mentioned an internal liquid for hollow threads or hoses.

In principle, any liquids can be used as the cooling liquid are used in which PVDF at the The temperature of the cooling liquid does not dissolve significantly and the no chemical change in the polymer blend cause.

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

Removal of the formed body from the cooling liquid can be done by known methods, wherein care must be taken to ensure that none of them are removed heavy mechanical stress on the mixture between Nozzle outlet and beginning to solidify becomes.

It may also be advantageous if the molded body at the same speed from the Withdraw cooling liquid, which the mixture at Has exit from the nozzle.

By the described method, in particular by the low mechanical stress until the beginning Solidification, pore-shaped PVDF moldings are obtained, the shape stability is good and the constant have good quality. Thread breaks and quality fluctuations  like uncontrollable defects in the form pore size and characteristic deviating from the target, are kept to a minimum.

Although it may be desirable in certain cases, in the pores of the PVDF molded body obtained Substances such as solvents and non-solvents, do not wash out, is the formed body in the normal case washed. This can be done by an extraction that continuously involved in the production of the shaped body connects or which is carried out discontinuously. After the extraction, the molded body is dried.

The moldings obtained have pores on each of them Surfaces on, d. H. also the inner surfaces of hollow threads or hoses have pore openings. Because of the wide scope, which regarding type and quantity of Solvent, type and amount of non-solvent, type, amount and throughput of the cooling liquid and the temperature control is given throughout the process pore size and characteristics of the PVDF molded body targeted and reproducible adjustment in wide areas. So it is possible to have average pore sizes of about 0.1 µ to get about 5 µ. The total pore volume leaves also in wide areas, e.g. B. on the weight ratio PVDF / mixing partner influence and lies about between 10 and 90%. Depending on the setting different process parameters different Characteristics of the pore system can be obtained. Depending on  The type and amount of the solvent and, if appropriate, the Non-solvent, type, amount and throughput of the cooling liquid and depending on the temperature control in the cooling device, can basically be two different types of Pore structures are obtained between which transitions possible are:

• a) a pore system with essentially spherical cavities, that through porous partitions from each other are separated. The pores of the partition walls have one smaller average diameter than the spherical Cavities;
• b) a three-dimensional network of pores that can only pass through narrow dividers, but not through dividing walls are separated.

In addition to these two ways of designing the Pore systems can affect this in another way will. This can be done primarily through variation the viscosity of the polymer mixture and the type and temperature the cooling liquid is either an isotropic or maintain anisotropic pore system. With an anisotropic Pore system have a pore size and / or structure Gradients in the direction from the surface into that Interior of the molded body. An anisotropic pore system is mainly obtained when the viscosity is relatively low Polymer blends are processed.

Leave the PVDF moldings produced according to the invention use versatile, z. B. for microfiltration  of aqueous solutions and of organic solvents. In many cases, PVDF membranes are an option, because they have exceptional resistance to you show oxidative attack resistant to an unusual large number of organic solvents are and their mechanical data only at nearby temperatures of the melting point drop significantly. In the case of filtration aqueous solutions, PVDF membranes can also be used there find where other polymers because of their vulnerability in strongly acidic or strongly alkaline pH range not can be used such. B. in the filtration of Hypochlorite solutions.

Hollow PVDF membranes with anisotropic structure, in which the Pore size decreases from the inside out, the use of a pre-filter for microfiltration do; prefiltration is often used, to trap large particles that result in membrane coverage and thus lead to a rapid drop in river. These large particles are found in the anisotropic membrane structures however, held in the large inner pores without to reduce the filtering area. The so-called So "dirt capacity" of the anisotropic PVDF membrane is correspondingly larger than in the case of isotropic structures.

The PVDF membranes produced according to the invention are suitable also for transmembrane distillation. In the Transmembrane distillation, in which the short diffusion distance (Membrane wall thickness) for evaporating aqueous Solutions is used is on the one hand  the heated solution to be concentrated on the membrane other side "cooling water". Wander through the vapor pressure difference Water as a gas from the hot to the cold side of the membrane. Here is the temperature resistance, the low surface tension of the PVDF, which is a breakthrough of watery Prevents solution, and the oxidative necessary for cleaning cycles Durability advantageously used.

The invention is illustrated by the following examples illustrates:

Example 1

A mixture of was placed in a heated glass vessel
20 parts by weight of polyvinylidene fluoride
(LV = 2.68, measured in dimethylformamide)
and 80 parts by weight of a solvent mixture
consisting of 37.5% glycerol triacetate (solvent) and 62.5% dioctyl adipate (non-solvent)
given. The polymer granules and the solvent mixture were brought to about 215 ° C. with vigorous stirring and in a nitrogen atmosphere. The granules were swollen at about 145 ° C. and a homogeneous, low-viscosity solution formed as the temperature rose.

Part of the solution prepared in this way 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 glycerin was used as the inner filling to form 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 the lower part of the tube and was pulled out of the outlet leg by means of a wire and placed on a take-off wheel. During the process, the water flowed through the device in the same direction as the polymer mixture at an average speed of 1 m / min. It was clearly observed how the thin polymer solution became milky after a short dwell time in the water at the beginning of phase separation and finally stabilized during solidification, so that it could be deflected without deformation and continuously drawn off at a speed of 20 m / min.

After extracting the liquid components at 50 ° C The thread became warm isopropanol at about 50 ° C. in vacuo dried.

Properties of the hollow fiber membrane obtained:

Outer diameter: 1.24 mm inner lumen: 0.88 mm max. Pore size: 0.58 µm transmembrane flow (isopropanol)
in ml / cm² · min at 0.1 bar: 0.95

The hollow thread was used to measure the maximum pore size dipped 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 were visible on the outside.  

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

where d _max. = maximum pore diameter, P _max. = Blowing point (bar).

To determine the flow of isopropanol, the hollow thread from charged with isopropanol tempered to 35 ° C and the Flow rate measured at 0.1 bar.

Microscopic examinations showed pore structure Openings on inner and outer walls.

An essentially spherical pore resulted exhibiting structure.

Example 2

There were 30 parts by weight of PVDF (LV = 2.68, measured in DMF) and 70 parts 2- (2-butoxy-ethoxy-) ethyl acetate at a temperature of approx. 155 ° C-160 ° C in a homogeneous medium solution Viscosity transferred. It started at about 120 ° C-130 ° C Observe sources of the granules.

The PVDF solution, whose solidification temperature is approx. 110 ° C was spun at about 150 ° C analogously to Example 1 and in a glycerol / water (1: 1) bath at 35 ° C cooled down. The extracted and dried hollow fiber had the following characteristics:

Outside diameter: 1.20 mm Inside volume: 0.80 mm max. Pore size: 0.94 µm transmembrane flow (isopropanol)
at 0.1 bar in ml / cm² · min: 0.57

It resulted in a pore system that essentially consists of a three-dimensional network of pores.

Claims (22)

1. The method according to patent application P 33 27 638.2 for the production of molded articles having pores by extruding a homogeneous, single-phase, liquid mixture of one or more polymers and one or more mixing partners liquid at the temperature of the preparation of the mixture, the mixture above room temperature in liquid state has a range of complete miscibility and a mixture gap and has a solidification range above room temperature, into a cooling device containing cooling liquid and pulling off the formed body, wherein the polymer / mixture partner mixture at a temperature above the mixture gap with an average linear velocity v 1 conveys a nozzle from top to bottom into a cooling liquid which does not or only slightly dissolves the polymer at the cooling temperature and which has a temperature below the solidification point, and the extruded mixture from the inlet tsstelle leads into the cooling liquid at least up to the point of the beginning 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 at least 20% lower than v ₁, the shaped body after the beginning Solidification of the polymer is redirected and passed from bottom to top through a second zone from which cooling liquid is drawn off, and the level of the cooling liquid 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 are constant holds, characterized in that the polymer polyvinylidene fluoride (PVDF) is used and the speed v ₂ is kept at least 25% lower than v ₁. 2. The method according to claim 1, characterized in that the PVDF molded body produced is a membrane in the form a hollow thread, tube or film. 3. The method according to claim 2, characterized in that in the case of the production of hollow fibers or hoses the inner lumen by introducing an inner filling liquid is generated at the temperature of generation the inner lumen is not a solvent for PVDF. 4. The method according to claim 3, characterized in that the inner filling liquid is glycerin or a liquid, which in the preparation of the mixture as Mix partner was used. 5. The method according to claim 2, characterized in that in the case of the production of hollow threads or hoses the inner lumen by introducing gaseous nitrogen is produced.   6. The method according to one or more of claims 1 to 5, characterized in that as a cooling device U-shaped tube is used. 7. The method according to one or more of claims 1 to 6, characterized in that the liquid level the cooling liquid at the exit point for the formed Shaped body is kept at the same level as at the point where the mixture enters the cooling liquid. 8. The method according to one or more of claims 1 to 7, characterized in that the cooling liquid the device opposed in the movement of the mixture Direction. 9. The method according to one or more of claims 1 to 7, characterized in that no continuous Cooling liquid is added, but only Losses are offset by the entrainment of cooling liquid through the molded body. 10. The method according to one or more of the claims 1 to 9, characterized in that between Nozzle and entry point of the mixture into the cooling liquid there is an air gap. 11. The method according to one or more of claims 1 to 10, characterized in that the cooling liquid is water, optionally with an addition of surfactant.   12. The method according to one or more of claims 1 to 11, characterized in that in the manufacture of the Mixture as solvent for PVDF one or more of the compounds glycerol triacetate, glycerol diacetate and 2- (2-butoxyethoxy -) - ethyl acetate is used or will. 13. The method according to one or more of claims 1 to 12, characterized in that in the manufacture of Mix in addition to at least one solvent at least one mixing partner is used, who at the temperature of the preparation of the mixture and its Promotion through the nozzle is a non-solvent for PVDF. 14. The method according to claim 13, characterized in that as a non-solvent di-n-octyl adipate or castor oil or a mixture thereof is used. 15. The method according to one or more of claims 1 to 14, characterized in that the proportion of PVDF in the mixture is 15-35% by weight. 16. The method according to one or more of claims 1 to 15, characterized in that the viscosity of the PVDF mixture at the temperature of their conveyance the nozzle has a value between 5 and 35 Pa · s. 17. Device for carrying out the method according to one or more of claims 1 to 16, characterized in that it contains a cooling device with the following components:

• a) an inlet opening for cooling liquid,
• b) an overflow device at which the cooling liquid can escape from the device,
• c) an inlet and an outlet opening for the PVDF mixture or the formed body,
• d) a channel-shaped zone surrounded by a wall, which adjoins the entry point of the PVDF mixture and in which the mixture can be guided from top to bottom,
• e) a deflection device which is located below the channel-shaped zone,
• f) a second zone following the deflection device in which the formed body can be guided from bottom to top,
• g) if necessary, an external thermostat.

18. Polyvinylidene fluoride (PVDF) molded articles attached to each of their Surface and interior pores can be produced by a method according to one or more of the Claims 1 to 16. 19. Use of one or more of the preceding Claims produced PVDF molded body for the Microfiltration of strongly acidic or strongly alkaline aqueous solutions. 20. Use of the one or more of the preceding Claims produced PVDF molded body for microfiltration of oxidizing media.   21. Use of one or more of the foregoing Claims produced PVDF molded body for microfiltration of aqueous hypochlorite solutions. 22. Use of the one or more of the preceding Claims produced PVDF molded body for transmembrane distillation.