JPH0369927B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is based on the general formula () CF ₂ =CF-O-R _f () [wherein R _f is perfluoroethyl-, perfluoro-n-propyl- or perfluoro-n-
Means a butyl group. ] Consisting of 0.004 to 0.075 mol% of polymerized units of perfluoroalkyl vinyl ether and polymerized tetrafluoroethylene units,
It relates to a free-flowing, granular, tetrafluoroethylene polymer raw polymer powder that cannot be processed from the melt, having a specific surface area of 0.5 to 4.5 m ² /g. The invention further relates to a method for producing such a powder by suspension polymerization. Polymer powders of this type are suitable for processing in ram extrusion methods without post-processing steps that alter the particle structure. Ram extrusion is a continuous powder smelt extrusion process for producing endless tubes and rods. In this process, polytetrafluoroethylene powder is introduced in repeated cycles by an automatic dosing device into a tube heated to half-melting temperature, compacted by a ram and in each case The section advances into the semi-melting tube. Under these conditions, the powder is semi-melted,
It is extruded into a uniform molded body. Polytetrafluoroethylene powder that can be well ram extruded has the best possible flow behavior.
That is, high bulk density and good powder flow index are required. Poor free-flowing powders lead to difficulties in automatic dosing of the product as well as uneven filling and compaction into the smelting tubes and hence unsatisfactory quality of smelting ram extrusion. Bring the finished product. Furthermore, the highest possible bulk density of the powder is worth the effort. This is because the productivity of the ram extruder can be increased with an increase in the bulk density of the powder used. In order to produce powder suitable for ram extrusion molding,
Usually, high molecular weight tetrafluoroethylene polymers are used as raw materials, which are obtainable by suspension polymerization in aqueous media in the presence of free radical-forming initiators. Tetrafluoroethylene polymer powders produced according to the emulsion polymerization process are generally not well suited for processing according to the ram extrusion process. This is because such powders are very soft and sensitive particles and have a low molecular weight compared to suspension polymers, resulting in extrudates with weak mechanical strength. Furthermore, emulsion polymers are more expensive to produce than suspension polymers and therefore cannot compete with the latter as raw materials for ram extrusion for economic reasons. Suspension polymerization of tetrafluoroethylene is generally carried out by
This is carried out by charging a polymerization reactor equipped with a stirring device with an aqueous medium in which the free-radical-forming catalyst, buffer substance and, if necessary, small amounts of fluorinated emulsifier are dissolved. The air in the remaining gas space of the reactor is carefully removed and tetrafluoroethylene is forced in to give a polymerization pressure of 4 to 30 bar. Gaseous tetrafluoroethylene is supplied while maintaining a constant polymerization pressure in proportion to the amount of polymer produced after the polymerization has begun. It is no surprise that the tetrafluoroethylene suspension polymer obtained directly from the polymerization reactor according to such conventional methods results in fibrous, irregular particles that are too coarse for most processing purposes. known to business owners. Such crude polymer powders are necessarily too large to obtain satisfactory extrudates by ram extrusion process due to their poor powder flow index, low bulk density and too large average particle size, usually greater than 1500 μm. accompanied by difficulties. The powder properties, in particular the bulk density and powder flow index of the unmodified suspension polymer powder of tetrafluoroethylene, are largely independent of the procedures in carrying out the reaction, such as stirring speed, temperature and pressure. For example, by increasing the stirring speed, it is indeed possible to reduce the average particle size of the polymer powder, but at the same time the powder becomes fibrous, the bulk density decreases and the powder flow index decreases. Therefore, the initial coarseness obtained in the polymerization reaction,
In order to obtain the necessary powder properties for smooth processing by automatically dispensing fibrous and poor free-flowing tetrafluoroethylene suspension polymer powder into a ram extruder. , it has become common practice to submit it to additional post-processing. Such post-treatment techniques include, for example, wet milling or wet cutting steps, micro-milling or semi-melting with subsequent agglomeration of the fine particles. Pre-semi-melted polytetrafluoroethylene powder is often used for the ram extrusion process.
This pre-semi-melting is carried out by heating the solid particles above the crystallite melting point of polytetrafluoroethylene, and the powder flow index of the pre-semi-melting powder is determined by the German patent application publication No.
This can be improved by post-treatment as described in No. 2744244. Pre-semi-melted powders are particularly
It is advantageously used to produce thin profiles at extrusion pressures above 250 bar.
Under high extrusion pressures, powders that have not been presemimelted often lead to so-called "double formations", in which solid dosing sections are visually visible as flaws. Used when producing thick profiles
In the range of extrusion pressures below 250 bar, the use of pre-semi-melted powders instead of non-pre-semi-melted powders offers no advantage in terms of extrudate properties. On the contrary, polymer powders that are not presemimelted lead to extrudates with better mechanical properties in this range. There is therefore no point in using pre-semi-smelt polytetrafluoroethylene powder to produce thick profiles by ram extrusion, which is relatively expensive due to the smelt-smelt step. Therefore, the tetrafluoroethylene suspension polymer obtained directly from the polymerization reactor is
If the polymerization process is successfully controlled as follows, it is a cost-effective raw material for producing thick profiles by ram extrusion. That is,
A bulk density as high as possible, a good powder flow index and particles that are neither too coarse nor too small are obtained directly and are therefore free from processing steps that improve the properties of the powder but make the product expensive, e.g. This would be possible if fine pulverization and the like that involve agglomeration of particles could be omitted. It is known to modify the properties of suspension polymers of tetrafluoroethylene by adding small amounts of copolymeric fluorinated unsaturated compounds to gaseous tetrafluoroethylene during polymerization. This type of process and the powder obtained therefrom are described in US Patent No.
3331822, British Patent No. 1116210 and German Patent Application No. 1940304,
No. 2325562 and No. 2416452. According to U.S. Pat. No. 3,331,822, a bulk density of 500 to 700 g/m and improved by copolymerizing 0.1 to 10% by weight of perfluoro-olefin with 3 to 4 carbon atoms and tetrafluoroethylene. Processes are known to obtain moldable powders with improved flow properties and a reduced tendency to coalesce. In this process, the properties of the powder are improved regardless of whether the perfluoroolefin is introduced from the beginning of the polymerization or is metered in in small amounts during the polymerization. Example 1 of this US patent specification
As you can see, the copolymer powder produced is 200μ
It has an average particle size of only m. Although such fine particles were an improvement at the time for the compression and melting process aimed at in the US patent, the average particle size was too small, and as a result, they were unsatisfactory for processing by ram extrusion. It has a certain powder flow index. Furthermore, it is known that incorporating even small amounts of perfluoroolefins, such as hexafluoropropylene, into tetrafluoroethylene polymers reduces the thermal stability of the polymers. This is explained in German Patent Application No. 1940304
or 2325562 and which is particularly suitable for compression semi-melting processing of the resulting moldings with avoidance of so-called "cold flow". The same applies to tetrafluoroethylene polymer powders modified with fluoroethylene. British Patent No. 1,116,210 discloses that tetrafluoroethylene is reacted with 0.003 to 1.5 mol % of a perfluoroethylene modifier, in which perfluoroolefin and perfluoro-[2-methylene-4-methyl -1,3-dioxolane] and also uses perfluoroalkyl vinyl ethers as modifiers. When small amounts of perfluoroalkyl vinyl ether are added, as with perfluorinated olefins, changes occur in the particle structure of the polymer, the fibrous content of the powder is reduced and the particles become granular and Takes a rounded shape. Such perfluoroalkyl vinyl ether,
In particular, the use of perfluoropropyl vinyl ether, compared to the use of perfluoroolefin, allows the produced polymer to have thermal stability comparable to that of pure tetrafluoroethylene polymer. It brings the advantage of Also, compared to pure polytetrafluoroethylene containing a small amount of modifier, the number of tetrafluoroethylene units
Polymers consisting of 0.01-0.2% by weight of perfluoropropyl vinyl ether units exhibit significantly reduced deformation under load, increased strength, low melt viscosity and high resistance to failure at break. It has the advantage of improved elongation rate. British Patent No. 1116210 (first text)
As can be seen from the examples, both the bulk density and the average particle diameter (d ₅₀ -value) of the powder obtained decrease as the amount of perfluoropropyl vinyl ether incorporated into the polymer increases. Such fine powders have insufficient bulk density and insufficient powder flow index and therefore cannot be processed successfully by ram extrusion methods. Tetrafluoroethylene polymer powders with a very low content (less than 0.0029 mol %) of the modifier perfluoropropyl vinyl ether are disclosed in DE 25 23 569 A1. After grinding, it is particularly suitable for producing compressed semi-melted blocks and cut sheets, but it is easily deformed under load, has a high melt viscosity and It has the disadvantage of low strength. The powder obtained directly from the polymerization reaction according to the method described in this publication has a bulk density that is too low and an average particle size that is too large to be used directly for ram extrusion. ing. Furthermore, DE 24 16 452 A1 describes the production of a tetrafluoroethylene polymer molding powder containing 0.02 to 0.26% by weight of perfluoroalkyl vinyl ether, in which suspension polymerization is carried out. It is also known to operate in the presence of 3 to 200 ppm of telogen-inactive fluorinated emulsifiers. The molding powders produced in this way, which should be particularly adapted to compression-smelting techniques, have an irregular fibrous particle morphology and a poor powder flow index in the raw polymer state, and cannot be easily processed by ram extrusion as is. Not suitable for molding. The object of the present invention is to provide a perfluoroalkyl compound in an amount that improves the final properties of the ram extrudate.
modified with vinyl ether, has improved flow properties (i.e. high bulk density, good powder flow index and very stable particles) and is particularly suitable for processing by ram extrusion. The goal is to create a granular powder that cannot be processed from the molten state. Yet another object of the present invention is to allow such properties to be imparted directly to the green polymer without the need for any post-processing steps that alter the particle structure prior to processing into ram extrudates. The objective is to find a method for producing such a powder. In order to solve such problems, the present inventors
average particle size d ₅₀ from 450 to 1400 μm, with removal of oversized particles with a particle size greater than 3000 μm;
Bulk density of at least 570g/, maximum 5.0s/50g
A raw tetrafluoroethylene polymer powder of the first mentioned type is presented having a powder flow index of and a particle stability of up to 5.5 seconds/50 g. The raw polymer powder of the present invention obtained directly from the polymerization process without post-processing steps affecting the particle structure has the general formula CF ₂ =CF-O-R _f [where R _f is perfluorinated]. ethyl-, perfluoro-n-propyl- or perfluoro-n-
Means a butyl group. ] Tetrafluoroethylene (hereinafter referred to as TFE) having incorporated polymerized units of perfluoroalkyl vinyl ether (hereinafter abbreviated as PAVE) represented by (abbreviated as ) suspension polymer (the remainder up to 100 mol% being incorporated polymerized TFE units). Particular preference is given to the perfluoro-n-propyl group. Mixtures of these ethers may also be present. Such suspension polymers of TFE are generally different from PAVE-rich copolymers which can be processed from the melt according to customary molding methods for thermoplastics. Rather, such suspension polymers, like unmodified polytetrafluoroethylene itself, cannot be processed from the melt and require special shaping methods - e.g.
Ram extrusion also belongs to the class of TFE polymers for which it has been developed. This TFE polymer is similar to unmodified polytetrafluoroethylene because the content of the above-mentioned PAVE copolymerizable monomer, which is usually called a modifier, is relatively small. It has high melt viscosity.
The melt viscosity is 1 to 200 Gpas, measured at 350°C, for the polymer on which the powder of the present invention is based.
Especially in the range of 10 to 100 Gpas. Slightly modified or unmodified TFE polymers have even higher melt viscosities in comparison. The TFE polymer constituting the powder of the present invention has a specific standard density in the range of 2.15 to 2.8, within which the density increases in proportion to the increasing content of modifier. As suspension polymers, the powders of the invention have the form of granules, in which they are prepared in the presence of an emulsifier in such a large amount that after the end of the polymerization they are distributed colloidally in the aqueous medium. It differs in principle from the emulsion polymers used. The so-called fine powders obtained by precipitation from such colloidal dispersions consist of aggregates of colloidal primary particles having an average particle size of about 0.1 to 0.5 μm. This cannot be molded according to the techniques of ram extrusion or compression melt processing, as known to those skilled in the art, nor can it be molded even when containing modifiers. The inventive green polymer powder of the modified TFE polymer as defined above has the following properties which were hitherto unachievable in combination: - from 450 to 1400 μm, especially suitable for ram extrusion;
Average particle size d ₅₀ in the range from 600 to 1200 μm, at least 570 g/ which can be up to −750 g/
, especially a bulk density of 620 to 750 g/5 s/50 g, which can go down to -3.0 s/50 g,
Especially powder flow index of 4.0 to 3.0 sec/50g, maximum 5.5 sec/50g which can go down to -3.0 sec/50g,
Particularly particle stability of 4.0-3.0 seconds/50g. Such powders are very suitable for automatic dosing into ram extrusion equipment due to their range of properties with respect to average particle size d ₅₀ , bulk density and powder flow index. The polymer powders of the invention yield ram extrudates with elongation at break of 380-560% at tear strengths of 28-32 N/mm ² . Extrudates with such high elongation at break with good tear strength values cannot be produced with conventional TFE polymer powders. TFE polymer powders with an average particle size of less than 450 μm, especially less than 300 μm, irrespective of their composition and origin, cannot be processed by the ram extrusion process due to the too small average particle size and the resulting insufficient powder flow index. It is not very suitable for processing according to Very coarse polymer powders containing large amounts of polymer particles with diameters greater than 3000 μm cannot be uniformly dosed and compressed when processed by ram extrusion, resulting in damaged extrusion. yield moldings. On the contrary, it has an average particle size of less than 450μm
TFE polymer powder is advantageous for compaction and melt processing. The non-agglomerated powder of the invention is a moldable powder consisting of known TFE suspension and emulsion polymers which have been subjected to post-treatment by so-called granulation or agglomeration steps in order to improve the flow behavior; It clearly depends on the particle stability. Particle stability is determined by the particle stability test described below in which mechanical stress is simulated by stirring the powder under specified conditions and then measuring the powder flow index of the powder subjected to mechanical stress. . When subjected to mechanical stresses that may arise during transport or during dosing into processing machines, the powder obtained from such an agglomeration process again partially or completely decomposes into primary particles, with the powder flow index decreasing. Although completely or partially lost, the structure of the polymer powder of the present invention is maintained due to its good particle stability. Surprisingly, the inventors have been able to obtain such superior powders of the invention directly from the suspension polymerization process, in which the raw polymer is conventionally removed from the aqueous polymerization medium. It was found to separate, wash, dry and separate a small proportion of oversized particles with a particle size >3000 μm. The raw polymer powder thus obtained can be used directly in ram extrusion. No post-processing steps to improve the properties of the powder, such as wet or dry agglomeration treatments, comminution, wet cutting or pre-smelting treatments, are necessary. This is expressed by the general formula () CF ₂ =CF-O-R _f () [wherein R _f is perfluoroethyl-, perfluoro-n-propyl-, or perfluoro-n-
Means a butyl group. ] A monomer mixture consisting of perfluoroalkyl vinyl ether and tetrafluoroethylene of the formula is fluorinated with conventional free radical-forming catalysts, buffer substances and optionally amounts of up to 40 ppm which are inert as telogens. as defined above by suspension polymerization under stirring in an aqueous medium in the presence of an emulsifier, followed by separation of the powdered polymer from the aqueous medium and removal of oversized particles with a particle size greater than 3000 μm. (a) in the form of a monomer mixture with tetrafluoroethylene; The concentration in the gas phase of the gaseous perfluoroalkyl vinyl ether of formula () is adjusted to a calculated value of 0.1 to 1.0% by volume for a period of time until a polymer solids content of 1.5% by weight relative to the aqueous medium is formed. and then the polymerization is continued by feeding tetrafluoroethylene and optionally a perfluoroalkyl vinyl ether of the formula, with the total amount of both monomers being fed between 0.004 and 0.075 mol % of the above formula. () a polymer containing polymerized perfluoroalkyl vinyl ether units and polymerized tetrafluoroethylene units should be formed, and (b) the polymerization should be carried out under a total pressure of 5 to 11 bar. This is made possible by the method according to the invention for producing the above-mentioned raw polymer powder, characterized in that it is carried out in the following manner. According to the method of the invention, the desired range of average particle size d ₅₀ , bulk density and powder flow index of the resulting raw polymer powder is determined by the selection of a combination of two process parameters: The calculated concentration of the aforementioned PAVE monomers in the TFE-containing monomer mixture from 0.1 to 1.0% by volume, in particular 0.1-0.45% by volume (this concentration is determined at the starting stage of the polymerization, i.e. 1.5% by weight of modified TFE relative to the weight of the aqueous polymerization medium)
Adjustments are made at the stage until the polymer is formed. ), maintenance of a relatively low polymerization pressure in the range -5 to 11 bar. This also includes the surprising finding that characteristic powder properties can be established during this TFE polymerization within a short period of time at the beginning of the polymerization process. In this case, the average particle size can be controlled within a very wide range depending on the calculated concentration of PAVE in the polymerization initiation stage. Even though relatively large concentration changes in the concentration range greater than 0.45% by volume and less than 1.0% by volume cause only small changes in the average particle size,
Relatively small changes in concentration within the concentration range of 0.1 to about 0.45% by volume result in large changes in average particle size. The relationship between the calculated concentration of PAVE and the average particle size of the raw polymer powder shows that the concentration change is TFE−pressure,
This is important regardless of whether it is caused by the amount of PAVE used to initiate the polymerization or by varying the size of the free gas space. Furthermore, it was surprising to find that other fluoroolefins known as modifiers, such as hexafluoropropylene and chlorofluoroethylene, did not exhibit such an effect. According to the method described in GB 1,116,210, the use of perfluoropropyl vinyl ether results in round and fine particles compared to unmodified TFE-suspended polymer powders. A product is obtained. However, as the amount of perfluoropropyl vinyl ether incorporated into the product increases, the average particle size and bulk density of the powder decreases to such an extent that a product with an insufficient powder flow index results. must be allowed.
In contrast, the method of the present invention allows control of powder properties even though the total amount of modifier in the entire polymer remains within the same range. In the sense of the present invention, the calculated concentration of PAVE in a TFE-containing monomer mixture in the gas phase is adjusted by introducing a certain amount of gaseous PAVE and TFE into the known free gas space of the polymerization vessel. (in volume % of PAVE in the total monomer mixture), assuming that both monomers behave as ideal gases and are completely vaporized and into the aqueous medium. Dissolution of is ignored. If the actual value of the concentration established in the gas space is determined analytically, for example by gas chromatographic analysis, this value is slightly reduced compared to the calculated concentration value. The concentration of gaseous PAVE in TFE at the initiation stage of the polymerization is determined by the desired degree of incorporation of PAVE.
Under the total amount (e.g. as specified above),
It can be adjusted in various ways: - by selecting the amount of water used for the polymerization and by selecting the free space volume in the polymerization reactor at a constant TFE pressure and a constant amount of PAVE; By selecting the TFE-pressure at a constant PAVE volume and a constant gas space volume, -at a constant gas space volume and at a constant TFE-pressure, the total amount of PAVE is determined as required. by dividing the total amount of PAVE required to incorporate into the total polymer, if greater than the starting concentration given. Adjusting the concentration by varying the free gas space present in the reactor has certain limitations for economic reasons. As a rule, efforts are made to keep the free gas space small and the water-filled volume large, in order to make the best possible use of the available reactor volume. The ratio of free gas space to the space filled with aqueous polymerization medium in the polymerization reactor can generally vary within the range from 0.2 to 1, in particular from 0.3 to 0.5. For a fixed amount of PAVE determined by the desired incorporation into the total polymer and a predetermined free gas space in the polymerization reactor, the PAVE concentration in the initiation phase is approximately equal to the TFE-pressure. Can be changed by selection. In this case, the selection of TFE single pressure means that the pressure is PAVE−
It must be done such that the total pressure obtained together with the pressure is within the limits of that second important reaction parameter. The total polymerization pressure should be in the range from 5 to 11 bar, in particular from 7 to 10 bar. This relatively low pressure range is another important criterion of the process of the invention, and it was not expected that this would affect the resulting properties of the suspension polymer. Finally, it is determined by the desired amount of incorporation.
If the total amount of PAVE is greater than the amount added due to the starting concentration, increase the total amount of PAVE added by 2.
May be divided into two portions. In this case: - a portion amount of 1.5% by weight, based on the aqueous medium, which serves to adjust the PAVE concentration in the polymerization initiation stage and therefore to determine the particle properties of the polymer powder;
added during the previous polymerization before the polymer solids are formed, and - fed continuously or in batches during the subsequent polymerization in a portion that has substantially no or only a slight influence on the properties of the particles; do. The addition of the amount of PAVE to adjust the concentration of the starting stage does not necessarily have to be done to the total amount or at or before the start of the polymerization. However, the concentration in the starting stage should be adjusted when a polymer solids content of 0.75% by weight based on the aqueous medium is formed and the concentration so adjusted is 1.5% by weight based on the aqueous medium. It should be maintained until polymer solids are formed. In an advantageous embodiment of the process of the invention, this initial stage concentration is adjusted at the beginning of the polymerization and maintained until a polymer solids content of 1.5% by weight is formed. In this case, this concentration may vary slightly provided that it remains within the concentration range according to the invention. Furthermore, the process of the present invention provides for the addition of PAVE in the monomer mixture with TFE by batchwise or continuous addition of PAVE during the remaining polymerization period following the initiation stage.
It is particularly advantageous to carry out the process in such a way that the concentration of PAVE remains essentially constant. The powder according to the invention has a content of polymerized PAVE units of from 0.004 to 0.075 mol%, in particular from 0.008 to 0.04 mol% (this is the case for perfluoro-n-propyl vinyl ether). (corresponding to a content of 0.01-0.2% by weight, in particular 0.02-0.1% by weight) in order to produce a TFE-polymer, with respect to TFE, during the entire polymerization period.
A total amount of PAVE of 0.015 to 0.15 mol %, in particular 0.015 to 0.075 mol %, which in the case of perfluoro-n-propyl vinyl ether is 0.04 to 0.4 mol %, in particular
(equivalent to 0.04-0.2% by weight) should be fed to the polymerization vessel. The PAVE is then consumed until the desired solids content is achieved, ie until the formation of a polymer. The process according to the invention is carried out at polymerization temperatures in the range from 50 to 120°C, in particular from 60 to 75°C. As catalysts used are the customary catalysts used for the suspension polymerization of tetrafluoroethylene polymers, in particular inorganic water-soluble peroxides or customary redox systems. Particular preference is given to inorganic persulfates, especially ammonium persulfate. The polymerization rate was determined by the aqueous medium of 1.
The usual range is from 50 to 300 g, in particular from 60 to 150 g, of polymer per hour. The PH value of the aqueous polymerization medium can be adjusted by adding customary buffer substances such as ammonium carbonate, ammonium phosphate or borax.
The PH-value should be in the range of 6 to 9, which is normal for suspension polymerization. In the process of the invention, the suspension polymerization is carried out to a solids content in the range from 15 to 50% by weight, in particular from 25 to 40% by weight (polymer solids), based on the weight of the aqueous polymerization medium. In this case, suspension polymers are obtained which have a specific surface area that is within the usual range for suspension polymers of TFE (ie from 0.5 to 3 m ² /g, measured according to the BET method). In this case, this value depends, inter alia, on the total amount of polymer solids formed relative to the weight of the aqueous polymer medium. Telogentically inert fluorinated emulsifiers, such as those described in DE 1109370, may optionally be added in small amounts (in the range 1 to 40 ppm). This makes it possible to increase the specific surface area to a value of 4.5 m ² /g.
Such fluorinated emulsifiers which are inert as telogens are in particular the alkali and ammonium salts of perfluorocarboxylic acids, such as in particular perfluorooctanoic acid. Another advantageous embodiment of the process of the invention is to carry out the suspension polymerization starting from 1.5 to 3% by weight of polymer solids relative to the aqueous medium and from 8 to 12% by weight of polymer solids. Under stirring, maintain a stirrer rotational speed corresponding to a specific power of 0.5 to 1.5 watts per aqueous medium (measured in the aqueous medium before the start of the polymerization) during the stages of the polymerization process up to the time when a minute fraction is formed. This is done by implementing the following measures. According to this particularly advantageous embodiment of the polymerization process according to the invention, if the stirring intensity is kept at a relatively low level at very specific stages of the polymerization process, the bulk density and fluidity of the resulting raw polymer powder can be improved. Behavior is further improved. This stirring intensity can be expressed by the specific input power of the stirrer, which is measured in the aqueous medium before the start of the polymerization. This stage of the suspension polymerization process, which is critical with respect to the stirring intensity in order to achieve particularly high bulk densities and very good flow behavior, is suitable for aqueous polymerization media.
Begins when 1.5% by weight polymer solids is achieved.
This ends when a solids content of 12% by weight relative to the aqueous medium has been formed. During this stage, is 0.5
Maintain a stirrer rotation speed corresponding to a specific power in the range ~1.5 W/1 (aqueous medium). Furthermore, a sufficient improvement in bulk density and flow behavior occurs at a polymer solids content of 1.5 which results in the onset of this critical stage with respect to stirring intensity.
The polymer solids content shifts from 12% to 8% by weight and/or shifts from 3% to 8% by weight.
% by weight, but this improvement decreases more and more the further this critical step is missed. After the end of this stage i.e. for aqueous medium
After forming a polymer solids content of 12% by weight, the stirring intensity is non-critical considering the bulk density and flow behavior of the resulting raw polymer powder. If desired, for example for reasons of improved heat removal and/or more homogeneous mixing, further stirring is carried out at the same stirrer rotational speed or the rotational speed is increased to a value corresponding to a specific power input of 20 W/. Good too. It is preferred to increase the power input (in particular by 0.5 to 2.5 W per aqueous medium compared to the critical stage). Before the onset of the critical stage, ie before 1.5% by weight of polymer solids is formed, there is likewise no criticality in maintaining the stirring intensity. In this period, it is possible to select a rotation speed corresponding to a higher, equal or lower input than the stirrer rotation speed characterized by the above range of 0.5 to 1.5 W/(aqueous medium). At this starting stage,
such that the corresponding power measured in the aqueous medium before the start of the polymerization is higher than that maintained during the critical stage (in particular higher by 0.5 to 2.5 W/(aqueous medium)).
It is advantageous to adjust the stirrer rotation speed. This is advantageous in further speeding up the initiation of polymerization.
In some cases, even higher stirring intensities are possible. The stirrer rotation speed resulting from the specific input is, of course,
It depends on the type and filling height of the polymerization vessel, on the shape and number of components in the polymerization vessel, such as flow cut-off means, temperature measuring devices, etc., and on the type and shape of the stirrer used. The specific power input for a given stirrer rotational speed or range thereof is determined in the aqueous medium before the start of the polymerization. The procedure for this measurement is as follows: the stirrer input is measured in an empty polymer container at various rotational speeds. The amount of aqueous medium required for suspension polymerization according to the method of the invention is then charged and the measurement repeated. Calculate the specific power (SP) for a specific stirrer rotational speed as follows: SP (W/) = P (when filled) (W) - P (when empty) (W)/filling volume () where P (when filled) and P (when empty) are the input power (W) measured at a specific rotational speed when the polymerization vessel is in a filled state and in an empty state. . When determining the relationship between specific input power and rotational speed for a particular polymerization reactor and a particular stirrer, maintenance of the specific input power required in accordance with the present invention can be achieved by adjusting the appropriate stirring program for the suspension polymerization process. In the said stage of the polymerization process, which can be easily carried out by making and which is critical for bulk density and flow behavior, the stirrer ratio according to the invention is within the above-mentioned range. Adjust the stirrer rotation speed to correspond to the input. For the polymerization, conventional reactors, flow barriers and stirrers are used. For example, anchor shape, disk shape,
Propeller-like, impeller-like or paddle-like stirrers can be used. Particular preference is given to reactors equipped with an impeller stirrer. This particularly advantageous procedure further improves the bulk density, powder flow index and particle stability of the raw polymer powder obtained. By maintaining the agitation conditions described above, the bulk density can be increased by about 100-130 g/m under an otherwise identical polymerization procedure. That is, in this case from 670 to 880 g/, and it is therefore also possible to obtain bulk densities in the range from 750 to 880 g/. In the case of this treatment, the measured powder flow index decreased by 0.5-1.5 seconds/50g, and decreased by 4.0-1.5 seconds/50g.
2.5 seconds/50g, especially values of 3.5 to 2.5 seconds/50g. Finally, for particle stability 4.0-2.5 seconds/50
g, in particular values of 3.5 to 2.5 seconds/50 g are achieved. Coarse components with a particle size of >3000 μm, which may occur in secondary amounts during the polymerization, are removed by sieving the polymer before further processing and before determining the parameters of the powder according to the invention. Separate and remove. In the method of the present invention, this coarse component is
At most 25% by weight, usually not more than 15% by weight. The values characterizing the powder produced according to the invention described in the specification and examples are measured according to the following measurement method: (1) Bulk density Coarse components with a particle size exceeding 3000 μm After separation, measurements are carried out according to DIN 53468. (2) Average particle size (d ₅₀ ) This measurement is carried out by sieve analysis according to DIN 53477 for a vibration time of 15 minutes using the sieves listed in the table, with measurements of the average particle size d ₅₀ >
Coarse components with a particle size of 3000 μm are ignored. (3) Powder flow index Coated with polytetrafluoroethylene,
An aluminum funnel with an inner diameter of 74 mm (upper side), an inner diameter of 12 mm (lower side) and a height of 89 mm was attached to a commercial vibrator such that the distance from the vibrator motor casing to the middle of the funnel was 90-100 mm. Fix it. Fill the funnel with 50g of product,
A vibrator with an amplitude of 0.5 to 1 mm is switched on and the time from opening the funnel opening until the funnel is completely empty is measured. The powder flow index increases as the outflow time decreases.
It's getting better. Before measuring powder flow index>
Remove coarse components of 3000μm. (4) Particle stability Fill an aluminum beaker with an internal diameter of 100 mm and a height of 150 mm with 50 g of powder and stir for 5 minutes under 1000 revolutions/min. 2
A stirrer with two blades is held by a pin and a recessed part in the bottom of the beaker. The distance from the lower edge of the stirring blade to the bottom is 1.5 mm. The stirring blades with a thickness of 1.5 mm, a width of 25 mm and a length of 46 mm are at an angle of 45° to the stirrer axis and at an angle of 90° to each other. The edges of the wings are slightly rounded. To avoid electrostatic charging, 0.1 g of aluminum oxide is added to the product before stirring. Also in this measurement, coarse components of >3000 μm are removed in advance. The powder flow index of the stirred product in an aluminum container is then determined as described under (3). The powder flow index obtained after applying mechanical stress is used as a measure of particle stability. Comparing the powder flow index before and after the agitation treatment reveals to what extent the particles are destroyed under mechanical stress. (5) Determination of PAVE content The PAVE content of the polymer produced according to the present invention is determined by subtracting the amount of monomeric PAVE remaining after polymerization in the reactor from the total amount fed to the reactor. It can be calculated on balance. Furthermore, a method for determining the content of perfluoropropylovinyl ether in polymers is described in detail in DE 24 16 452 A1. An IR-spectroscopic analysis method is used. (6) Specific surface area This is defined by S. Brunaur, P.
Following the BET method of Emmet and E. Teller [J.Amer.
Chem.Soc., 60 (1938), p. 309] Areatron type apparatus [Manufacturer:
Measured at Leybold in Cologne. (7) Specific standard density This measurement is carried out on a specimen with a thickness of 4 to 7 mm, which is scraped from a ram-extruded specimen.
This specimen was heated to 380°C for approximately 30 minutes, then 60°C
Cool down to 300°C at a rate of °C/hour. Other points are carried out in accordance with ASTM1457-56T. (8) Specific melt viscosity The viscosity of the melt is measured at a temperature of 350° C. on a specimen having a width of 0.25 cm, a thickness of 0.65 cm and a length of 5 cm. The basic principles of the measurement method for measuring the elongation rate under known tensile stress were described by Ajroldi et al. in J. Appl. Polym.
Sci. 14 (1970), pages 79 onwards. (9) Tensile strength and elongation at break Measure according to ASTM1457-62T. Examples 1 to 19, Comparative Examples A to E Examples 1 to 19 and Comparative Examples A to E are carried out according to the following formulations. The polymerization is carried out in an enameled polymerization reactor with a total volume of 400 ml. The polymerization reactor is equipped with a vane stirrer and a two-armed flow cut-off. 210 g of demineralized water containing molten 35 g of ammonium carbonate is introduced into the reactor. Bring the reactor contents to a temperature of 70°C. The remaining gas space is evacuated by several injections of nitrogen. After that, TFE is press-fitted and the pressure is released. Add the calculated amount of PPVE needed to adjust the desired perfluoropropyl vinyl ether (PPVE) concentration in TFE for the polymerization initiation stage. Thereafter, TFE is press-fitted to the desired polymerization pressure. Set the stirrer rotation speed to 130 revolutions/minute [this is
1.6W/measured in aqueous medium before polymerization starts
(corresponds to the stirrer ratio input of the aqueous medium). The ammonium persulfate used as catalyst is then pumped into the polymerization reactor dissolved in 100 ml of demineralized water. After the start of polymerization, the polymerization pressure is automatically maintained constant by supplying fresh TFE and the polymerization temperature by cooling. The amount of TFE fed is measured and recorded by an orifice plate. The amount of PPVE initially introduced was
If the total amount of PPVE is not reached, which depends on the desired amount of PPVE incorporated, the remaining amount is introduced at another stage of the polymerization. When the desired polymer weight, calculated for the aqueous medium used, has been achieved, the reaction is stopped by releasing the TFE. Tetrafluoroethylene remaining in the gas space of the polymerization reactor is purged with nitrogen. The resulting polymer powder is separated from the aqueous polymerization medium, washed with 100 ml of demineralization and dried for 10 hours at 250° C. in an air-circulating drying chamber. Coarse particles with a particle size of more than 3000 μm, which are produced in secondary quantities during the polymerization, are removed by a sieve. Table 1 shows the polymerization conditions for Examples 1 to 19 and Comparative Examples A to E. Differences from the above prescription and additional conditions are noted in footnotes. Table 1 lists the properties of the TFE polymer powders produced. As noted in the footnotes to Table 1, Example 18 uses perfluoroethyl vinyl ether in place of PPVE, Comparative Examples C and D use hexafluoropropylene, and Comparative Example E uses chlorotrifluoropropylene. Use ethylene. The symbol "∞" in the "Powder Flow Index" column in the table means that the powder flow stopped during the measurement. In this case, an automatic dosing device operating under test conditions is prevented from operating. The powders obtained according to Comparative Examples A to E consist exclusively of coarse particles >3000 μm. Therefore, this cannot be excluded in this case.

【table】

[Table] The resulting polymer solids

Form content (weight%)

Use of ammonium persulfate 3.8 2.6
1.9 3.0 3.8 3.8
6.0
Dose (g)

Polymerization time (hours) 4.0 4.1
4.6 4.0 3.5 3.6
4.7

Claims (1)

[Claims] 1 General formula () CF ₂ = CF-O-R _f () [In the formula, R _f means perfluoroethyl-, perfluoro-n-propyl-, or perfluoro-n-butyl group. do. ] Consists of 0.004 to 0.075 mol% of polymerized units of perfluoroalkyl vinyl ether represented by and polymerized tetrafluoroethylene units, and 0.5 to 4.5
A free-flowing, granular, tetrafluoroethylene polymer raw material obtained directly from polymerization without post-processing, including structuring of the particles, having a specific surface area of m ² /g and which cannot be processed from the melt. In polymer powders, average particle size from 450 to 1400 μm, not including too large particles with a particle size larger than 3000 μm.
d ₅₀ , bulk density of at least 570 g/, maximum 5 s/
Polymer powder as described above, characterized in that it has a powder flow index of 50g and a particle stability of up to 5.5 seconds/50g. 2. A free-flowing, granular, raw tetrafluoroethylene polymer powder according to claim 1, which cannot be processed from the melt, having a bulk density of 620 to 750 g/g. 3. A free-flowing, granular tetrafluoroethylene polymer raw polymer powder according to claim 1, which cannot be processed from the melt, having a bulk density of 670 to 880 g/g. 4. A free-flowing, granular tetrafluoroethylene polymer raw polymer powder according to claim 1, which cannot be processed from the melt, having a bulk density of 750 to 880 g/g. 5 Powder flow index of 4.0~3.0 seconds/50g and 4.0~
A free-flowing, granular tetrafluoroethylene polymer raw polymer powder, which is not processable from the melt, as claimed in claim 1, having a particle stability of 3.0 seconds/50 g. 6 Powder flow index of 4.0~2.5 seconds/50g and 4.0~
A free-flowing, granular tetrafluoroethylene polymer raw polymer powder, which is not processable from the melt, as claimed in claim 1, having a particle stability of 2.5 seconds/50 g. 7 Powder flow index of 3.5~2.5 seconds/50g and 3.5~
A free-flowing, granular tetrafluoroethylene polymer raw polymer powder, which is not processable from the melt, as claimed in claim 1, having a particle stability of 2.5 seconds/50 g. 8. A free-flowing, granular tetrafluoroethylene polymer raw polymer powder according to claim 1, which cannot be processed from the melt, having an average particle size d ₅₀ of from 600 to 1200 μm. 9 Claim 1 in which the polymerized unit is a unit of perfluoro-n-propyl vinyl ether
Free-flowing, granular tetrafluoroethylene polymer raw polymer powder, which cannot be processed from the melt, as described in Section 1. 10 General Formula () CF ₂ =CF-O-R _f () [In the formula, R _f means perfluoroethyl-, perfluoro-n-propyl- or perfluoro-n-butyl group. ] A monomer mixture consisting of perfluoroalkyl vinyl ether and tetrafluoroethylene of the formula is suspension polymerized with stirring in an aqueous medium in the presence of customary free radical-forming catalysts and buffer substances, followed by By separating the powdered polymer from the aqueous medium and removing oversized particles with a particle size greater than 3000 μm, an average particle size d ₅₀ of from 450 to 1400 μm, a bulk density of at least 570 g/m;
Up to 5 seconds/50g powder flow index and up to 5.5 seconds/
With a particle stability of 50 g, the polymerized units of the above perfluoroalkyl vinyl ether are 0.004 to 0.075.
A free-flowing, granular tetrafluoroethylene polymer raw polymer that cannot be processed from the melt state, consisting of mol % and polymerized tetrafluoroethylene units and having a specific surface area of 0.5 to 4.5 m ² /g. In producing the combined powder, (a) the concentration in the gas phase of the gaseous perfluoroalkyl vinyl ether of formula () in the monomer mixture state with tetrafluoroethylene is 1.5 with respect to the aqueous medium; The polymer solids content is set at 0.1-1.0% by volume in the initial stage until a weight percent polymer solids is formed, and then the polymerization is carried out under the feed of tetrafluoroethylene and optionally a perfluoroalkyl vinyl ether of the formula Then, the total amount of both monomers supplied is 0.004 to 0.075 mol%.
It should be determined that a polymer containing polymerized perfluoroalkyl vinyl ether units and polymerized tetrafluoroethylene units of the above formula () is formed, and (b) the polymerization is carried out at a total pressure of 5 to 11 bar. The method for producing the raw polymer powder described above, characterized in that the method is carried out under the following conditions. 11 Suspension polymerization as an inert telogen
11. A process according to claim 10, carried out in the presence of a fluorinated emulsifier in an amount up to 40 ppm. 12. The method according to claim 10, wherein the concentration of the gaseous perfluoroalkyl vinyl ether of formula () is a calculated value of 0.1 to 0.45% by volume. 13 starting when forming a polymer solids content of 1.5 to 3% by weight and 8 to 12% by weight of the aqueous medium.
During the phase during the polymerization process, up to the time when a polymer solids of 11. A method as claimed in claim 10 in which the method corresponds to: 14 From the start of the suspension polymerization until a polymer solids content of 1.5% by weight based on the aqueous medium is formed.
Claim 1 carried out at a stirrer rotational speed corresponding to a higher specific input (measured in the aqueous medium before the start of the polymerization) than that used in the phase defined in claim 13 during the polymerization process. The method described in item 13.