Process for the preparation of tetrafluoroethylene polymer in aqueous suspension

In the suspension polymerization of tetrafluoroethylene a mixture of tetrafluoroethylene and an inert gas is charged under pressure before starting the polymerization, the total pressure being 5 to 50 bar and the concentration of tetrafluoroethylene in this mixture being 30 to 70 mol %. Said concentration is kept within this range by an appropriate feed of tetrafluoroethylene during the polymerization. The granular PTFE powder obtained is distinguished by an improved grain structure and grindability.

The invention relates to a process for the preparation of granular 
tetrafluoroethylene polymer which cannot be processed from the melt, by 
suspension polymerization of tetrafluoroethylene and 0 to 0.6 mol % of a 
modifying comonomer in an aqueous phase in the presence of free-radical 
forming initiators, a mixture of tetrafluoroethylene and an inert gas 
being charged under pressure before starting the polymerization. 
The suspension polymerization of tetrafluoroethylene (TFE) in aqueous phase 
has been known for a long time (U.S. Pat. No. 2,393,967). It is carried 
out in the presence of water-soluble initiators, such as, for example, 
alkali metal or ammonium persulfates, percarbonates, perphosphates or 
perborates, or water-soluble redox initiators. Buffers and precipitants 
are also usually added in the course of the polymerization. The air in the 
remaining gas space of the reactor is carefully removed and TFE is 
charged, it being possible for the polymerization pressure to be between 4 
and 30 bar. After the polymerization has started, gaseous TFE 
corresponding to the amount of polymer formed is usually fed in while the 
polymerization pressure is kept constant. 
The TFE polymers obtained in this manner by suspension polymerization are 
usually described as granular PTFE. They contain comonomers only as 
so-called modifying agents, i.e. to such a small extent that the TFE 
polymer retains the characteristic of PTFE homopolymer, of not being 
processible from the melt ("non-melt-fabricable"). Granular PTFE differs 
in principle from the second type of PTFE powders, the so-called fine 
powders, which are obtained by polymerization in the presence of 
fluorinated emulsifiers to give aqueous colloidal dispersions, which are 
subsequently coagulated, in respect of its grain structure, specific 
surface area, its powder properties and processing properties and its 
possible uses. 
The present invention is concerned with the preparation and working up of 
granular PTFE. It is known to those skilled in the art that the suspension 
polymers of TFE obtained direct from the polymerization reactor by 
customary processes are produced in the form of fibrous, irregular 
particles which are too coarse for most processing purposes. Crude polymer 
powders of this type can only be processed further with the greatest 
difficulty because of their poor flowability, their low bulk density and 
their excessively large average particle diameter, which as a rule is over 
1500 .mu.m. In most cases such crude polymers are subjected to grinding 
down to average particle diameters of &lt;200 .mu.m, as a result of which the 
mechanical and electrical properties of shaped articles produced therefrom 
are improved. 
Particularly in cases where automatic metering equipment is employed for 
the customary further processing methods of granular PTFE by the so-called 
compression/sinter technique or by ram extrusion, as is generally 
customary nowadays, trouble-free metering with crude polymer powder is not 
possible. The poor flowability results in frequent blockages in the 
material flow of the metering, and the excessively low bulk density 
hinders adequate utilization of the capacity of the equipment, since too 
little material can be introduced per working cycle. The excessively 
coarse grain causes excessively high porosity and inadequate smoothness of 
the surface of articles shaped therefrom. 
It has therefore been known for a long time to improve the processibility 
of granular PTFE molding powders of this type by so-called agglomeration 
processes in agitated liquid media under the action of mechanical forces. 
Liquid agglomeration media of this type which have been described are 
water, organic liquids capable of wetting PTFE, amino-substituted and/or 
hydroxyl-substituted alkanes or mixtures of water and organic liquids 
which are substantially insoluble in water. The requirement for an 
agglomeration process of this type is prior grinding of the crude polymer 
obtained to an average particle diameter of &lt;200 mm. In a customary 
process, such as is described, for example, in U.S. Pat. No. 2,936,301, a 
grinding of this type is advantageously carried out in a gas jet mill, but 
the flow properties achieved with this are not adequate for automatic 
metering. Before the grinding process it is necessary to carry out a 
washing and drying process, and this is hindered to an extraordinary 
degree by the poor handling properties. For this reason it has become 
customary to precede the drying and fine grinding of the granular TFE 
crude polymer by wet grinding by means of a cutting device, in the course 
of which the washing of the crude polymer and the removal of the 
polymerization auxiliaries are effected at the same time. However, this 
wet grinding stage constitutes a measure which is time-consuming and 
susceptible to trouble and which one would be glad to dispense with. The 
object of the present invention is to make this possible by imparting an 
improved grain structure to the crude polymer. 
This object is achieved by means of a process of the type mentioned 
initially, wherein the total pressure of the charged mixture is 5 to 50 
bar and the concentration of the tetrafluoroethylene in this mixture is 30 
to 70 mol %, said concentration being kept within this range by an 
appropriate feed of tetrafluoroethylene during the polymerization. 
The suspension polymerization of the process according to the invention is 
carried out in an aqueous phase with stirring and under conditions which 
are otherwise customary. The known free-radical forming and water-soluble 
initiators can be used, in particular the alkali metal and ammonium salts 
of persulfates and percarbonates, and also water-soluble peroxides, acyl 
peroxides or alkali metal permanganates. It is also possible to employ 
redox systems containing one of the abovementioned peroxidic compounds, in 
particular persulfate, as the oxidizing component and containing a 
reducing component, such as bisulfite, hydrazine, dithionite or a 
water-soluble nitrogen compound which yields a diimine, such as, for 
example, azodicarboxylic acid and salts thereof or azodicarboxamide. The 
polymerization can be carried out either in a slightly acid medium or in a 
slightly alkaline medium. If permanganates are employed, it is 
advantageous to use an acid medium; in the case of all the other 
initiators it is advantageous to use an alkaline medium. The 
polymerization can, if desired, be carried out in the presence of small 
amounts of buffer substances, such as, in particular, ammonium salts, for 
example ammonium carbamate, ammonium carbonate and ammonium oxalate. It is 
also possible, if appropriate, for known precipitants, such as, for 
example, borax or inorganic, water-soluble phosphates, to be present. In 
order to facilitate the initial decomposition of redox systems, it is also 
possible, if appropriate, for small amounts of heavy metal salts to be 
present in concentrations of 1.times.10.sup.-5 to 40.times.10.sup.-5 % by 
weight, relative to the total liquor, in the form of salts of, for 
example, bivalent copper, bivalent or trivalent iron or trivalent 
chromium. 
The polymerization of the TFE can be carried out as a pure 
homopolymerization, or small amounts of a modifying comonomer, namely 0 to 
0.6 mol %, preferably 0 to 0.4 mol % and, in particular--if present--0.05 
to 0.4 mol %, relative to the TFE in the gas space, can be present in the 
polymerization. The amount of such a modifying comonomer thus incorporated 
into the polymer is made sufficiently small for the characteristic of 
PTFE--namely not to be capable of shaping from the melt--to be retained. 
Modifying comonomers are perfluorinated olefins and perfluoroalkyl 
perfluorovinyl ethers, including those which are substituted in the 
.omega.-position by H, preferably hexafluoropropylene and perfluoropropyl 
perfluorovinyl ether and also chlorotrifluoroethylene. 
If appropriate, the aqueous polymerization medium also advantageously 
contains small amounts of perfluorinated emulsifiers which are inactive to 
polymerization, such as, for example, salts of perfluorocarboxylic acids. 
The concentration of emulsifiers of this type should, however, not exceed 
30 ppm and is preferably below 20 ppm. This amount is made sufficiently 
small for the polymer produced not to remain as a colloidal dispersion in 
the aqueous medium. 
The suspension polymerization is carried out at a temperature of 5.degree. 
to 90.degree. C., preferably 10.degree. to 40.degree. C., this temperature 
being kept essentially constant during the course of the polymerization. 
The polymerization of the process according to the invention is carried out 
in the presence of an inert gas. In this regard the total pressure built 
up by charging TFE and inert gas before the start of the polymerization 
should be 5 to 50 bar, preferably 5 to 30 bar, the concentration of TFE in 
the mixture charged being 30 to 70 mol %, preferably 30 to 65 mol %, of 
the total mixture. As is customary, the reaction vessel charged with the 
aqueous medium containing the polymerization ingredients is freed 
carefully from air or oxygen beforehand by repeated alternate evacuation 
and flushing with nitrogen. 
When the polymerization has been initiated, TFE is fed in in an amount such 
that the concentration of TFE, relative to the gas mixture, is kept within 
said range of 30 to 70 mol %, preferably 30 to 65 mol %. In a preferred 
embodiment of the process according to the invention TFE is fed in 
essentially at the rate in which it is consumed by the polymer formed, the 
total pressure in the reaction vessel increasing in step with the decrease 
in the free gas space, but remaining within the limits indicated and the 
TFE concentration remaining essentially constant. In another preferred 
variant of the process according to the invention the feed of TFE is 
regulated in such a way that the total pressure during the period of 
polymerization remains constant and the TFE concentration decreases during 
this time, but remains within the limits given above. 
Within the scope of the process according to the invention inert gases are 
any gaseous elements or compounds which are inert toward TFE, if 
appropriate, also toward modifying comonomers present, and toward the 
ingredients of the TFE polymerization, and which do not interfere with the 
polymerization. Preferably these are noble gases, such as helium, argon, 
krypton or neon, carbon dioxide or perfluorocarbon compounds, such as 
perfluoromethane or perfluoroethane, but especially nitrogen. 
It should be noted that a polymerization of TFE with the addition of inert 
gases is mentioned in U.S. Pat. No. 2,394,293. The suspension 
polymerization of TFE described there is stated to be carried out in an 
aqueous phase under extremely high pressures. Pressures up to 35.2 bar 
(500 lbs./sq. in) are described as still effective, but the pressures 
actually used are substantially higher, namely within the range from 70 to 
280 bar. In this regard these pressures are to be understood expressly as 
the TFE partial pressure; inert gas which may have been added is described 
simply and solely as an agent for increasing the pressure further. The 
addition of inert gas is not of critical importance. The pressures 
mentioned are the initial pressures; a feed of TFE during the 
polymerization is not envisaged. 
Nothing is stated concerning a specific concentration of TFE to be 
maintained or concerning an effect on the polymer formed. It was therefore 
very surprising that in the process according to the invention the grain 
structure of the resulting TFE polymerization is influenced in an 
extremely advantageous manner by the addition of inert gas in a specific 
proportion. A suspension polymer of TFE prepared by a customary method has 
a greatly preponderant proportion of particles which have a fibre-like or 
filament-like shape or are particles having filament-like appendages, and 
this type of particle is responsible for the poor handling properties of 
the powder in the drying and grinding process. As a result of this the 
flow through the dryer is impeded in the drying process and hence the 
continuous feed to the gas jet mill is impeded, so that on-spec grinding 
poses great problems. Hence, the prior addition of a wet grinding stage is 
indispensable in continuous working up processes. 
The product prepared by the process according to the invention has 
decidedly fewer filament-like particles or appendages of this type and has 
a considerably more uniform particle size, and thus is appreciably more 
easy to handle in working up. The result of this is that the TFE polymer 
when discharged flows more easily from the kettle and is easier to wash, 
and that it is possible to dispense completely with the wet grinding stage 
without problems arising in the drying process and in subsequent fine 
grinding in the gas jet mill. 
The grindability of the TFE polymer prepared by the process according to 
the invention is also decidedly improved. Under the same grinding 
conditions, using a gas jet mill, a lower proportion of coarse material is 
observed in the case of the products prepared by the process according to 
the invention, as is evident from Table 2. 
If an impact mill, for example a Zirkoplex sifter mill made by Alpine, is 
used, the improved grindability can be seen from the fact that, for the 
same energy consumption of the mill, the throughput of on-specification 
product which can be achieved per unit time is markedly higher for the 
product according to the invention. 
Furthermore, the variant of the process according to the invention in which 
(with a decreasing TFE concentration) the total pressure is kept constant, 
brings a further advantage: the molecular weight distribution of the TFE 
polymer formed is considerably broader, and this, as is known, has a 
favorable effect on the properties of products shaped therefrom. 
The invention is illustrated by means of the following examples.

EXAMPLE 1 
100 l of demineralized water containing 6 g of ammonium oxalate in solution 
are charged to a 200 l enamelled kettle. The kettle is freed from any 
oxygen present by alternate evacuation and flushing with repurified 
nitrogen, and the temperature is adjusted to 15.degree. C. 7 bar pressure 
of nitrogen and 6 bar pressure of TFE, corresponding to a total pressure 
of 13 bar, are then charged successively. The polymerization is initiated 
by a single addition of 150 mg of KMnO.sub.4, dissolved in 100 ml of 
degassed water. TFE is fed in continuously during the polymerization at 
such a rate that the concentration of TFE in the gas space, which is 
reduced by the formation of polymer, in the kettle is not substantially 
altered. This is achieved by increasing the desired value of the TFE 
pressure regulator in stages in accordance with the decrease in gas 
volume. The decrease in gas volume is calculated from the amount of TFE 
which has been admitted into the kettle and measured cumulatively and from 
the density (.rho.=2.3 g/cm.sup.3) of the PTFE formed. Thus the total 
pressure is thereby increased continuously during the polymerization, 
specifically after the formation of every 1 kg of PTFE. The composition of 
the gas phase is then 54 mol % of N.sub.2 and 46 mol % of TFE. This 
composition of the gas phase remains essentially constant during the whole 
duration of polymerization. When the amount admitted reaches 40 kg, the 
total pressure is about 1.2 times the initial pressure. After said 
admitted amount of 40 kg has been reached, the polymerization is 
terminated by shutting the TFE valve and releasing the pressure. 
The kettle is flushed several times with N.sub.2 and the polymer is thereby 
degassed. Water is roughly separated from the suspension via a sieve. 
Here, as in all the following examples, the crude polymer is dried at 
220.degree. C. in a circulating air drying cabinet. 
EXAMPLE 2 
The kettle is charged as described in Example 1 and, after being evacuated 
and flushed with nitrogen, is placed under a pressure of 7 bar of N.sub.2 
and 6 bar of TFE, and the polymerization is initiated similarly with 150 
mg of KMnO.sub.4. As distinct from Example 1, however, the total pressure 
of 13 bar is kept constant during the whole polymerization by controlled 
feeding in of TFE, by adjusting the desired value of the pressure 
regulation to this value. After the amount admitted has reached 40 kg the 
polymerization is terminated. 
In this test the gas phase is composed of 54 mol % of N.sub.2 and 46 mol % 
of TFE at the start of the polymerization and of 65 mol % of N.sub.2 and 
35 mol % of TFE at the end of the polymerization. 
EXAMPLE 3 
Procedure as in Example 1, but highly purified carbon dioxide is used as 
the inert gas instead of nitrogen. 
EXAMPLE 4 
Procedure as in Example 2, but highly purified carbon dioxide is used 
instead of nitrogen. 
EXAMPLE 5 
Procedure as in Example 1, but argon is used as the inert gas instead of 
nitrogen. 
EXAMPLE 6 
Procedure as in Example 1, but helium is used as the inert gas instead of 
nitrogen. 
EXAMPLE 7 
Comparison example 
The kettle is charged and blanketed with inert gas as described in Example 
1. After evacuation to remove the flushing nitrogen, a pressure of 6 bar 
of TFE is charged and the polymerization is initiated by a single addition 
of 150 mg of potassium permanganate. The pressure is kept constant for the 
whole period of polymerization and is terminated when the amount admitted 
has reached 40 kg. 
EXAMPLE 8 
Comparison example 
The test is carried out as in Example 2, with the total pressure being kept 
constant, but with the difference that at the start of the polymerization 
the composition of the gas phase is 25 mol % of N.sub.2 and 75 mol % of 
TFE at a total pressure of 8 bar. At the end of the reaction the 
composition of the gas phase is 30 mol % of N.sub.2 and 70 mol % of TFE. 
EXAMPLE 9 
The 200 l kettle is charged with 100 l of demineralized water in which 40 g 
of (NH.sub.4).sub.2 CO.sub.3 have been dissolved and is blanketed with 
inert gas by alternate evacuation and flushing with N.sub.2, and the 
temperature is adjusted to 70.degree. C. 10 g of PPVE are then in and a 
pressure of 5.5 bar of N.sub.2 and 10 bar of TFE is charged. The 
polymerization is initiated by a single addition of 3 g of ammonium 
persulfate, dissolved in 100 ml of degassed water. PPVE is metered in in 
stages during the polymerization, after every 1 kg of PTFE, specifically 
about 1.5 g per kg of PTFE, making a total of 60 g. 
A total pressure is kept constant during the whole time of polymerization. 
After the amount admitted has reached 40 kg the polymerization is 
terminated by discontinuing the addition of TFE and releasing the 
pressure. 
At the start of the polymerization the composition of the gas phase is 64.5 
mol % of TFE and 35.5 mol % of N.sub.2 ; at the end of the polymerization 
it is 57.4 mol % of TFE and 42 6 mol % of N.sub.2. 
EXAMPLE 10 
As Example 9, with the difference that 8 bar of N.sub.2 and 8 bar of TFE 
are charged into the kettle at the start of polymerization. Here too the 
total pressure of 16 bar is kept constant for the duration of 
polymerization, and the polymerization is terminated when the amount 
admitted is 40 kg. The composition of the gas phase at the start of 
polymerization is 50 mol % of TFE and 50 mol % of N.sub.2 ; at the end of 
polymerization it is 40 mol % of TFE and 60 mol % of N.sub.2. 
EXAMPLE 11 
Comparison example 
The polymerization is carried out as described in Example 9, with the 
difference that no nitrogen is charged, so that the polymerization is 
carried out under a constant TFE pressure of 10 bar. 
EXAMPLE 12 
Comparative Example 
Procedure as in Example 11, but with the difference that the polymerization 
is carried out under a constant TFE pressure of 8 bar. 
The dried crude polymer is subjected to a dry sieve analysis in accordance 
with ASTM Specification D 1457-88. A sieve setup made by Haver & Boecker, 
Model Haver EML 200-89 having a set of sieves of 3000, 2000, 1800, 1400, 
1000, 800, 600, 400 and 200 .mu.m is used in this analysis. 
The results in Table 1 are listed in cumulative form in accordance with 
ASTM Standard Specification D 1457-88. These results make it possible to 
assess the effect of the particle size distribution very simply via the 
d.sub.50 value, which is also shown. The reduction in this value in the 
case of the polymers obtained by the process according to the invention 
rests mainly on the markedly reduced proportion of fibrous particles in 
the crude polymer. 
The particle size distribution and the particle shape can be influenced to 
a certain extent by the particular conditions of stirring and by the 
roughness of the wall surface of the reaction vessel. Therefore all the 
tests listed in Table 1 were carried out in the same kettle and with the 
same stirring conditions. 
The dried crude polymer is also ground, under identical grinding conditions 
for all the examples and comparison examples listed here, without prior 
wet grinding via a metering hopper through a metering screw into an air 
jet mill made by Alpine, model 315 AS. The grinding conditions are room 
temperature and an input pressure of 5 bar. The metering rate is 14.5 kg/h 
in Examples 1 to 8, and 8.5 kg/h in Examples 9 to 12. The lower metering 
rate in Examples 9 to 12 is necessary, because, as is known, the modified 
TFE suspension polymers are basically more difficult to grind than the 
polymers which have been prepared without the addition of modifying 
agents. In each case 15 kg of crude polymer are ground. 
The ground product is subjected to a wet sieve analysis in accordance with 
said ASTM Standard Specification. An instrument made by Haver & Boecker 
having a set of sieves of 100, 75, 50 and 33 .mu.m is used in this 
analysis. 
The results of this wet sieving are listed in Table 2, also in cumulative 
form. This table documents the improved grindability of the products of 
the process according to the invention through the increase in the 
fraction &lt;33 .mu.m and through the reduction in the fractions &gt;50 .mu.m. 
TABLE 1 
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Integral particle size distribution of the crude polymer 
Example 
&gt;3000 
&gt;2000 
&gt;1400 
&gt;1000 
&gt;800 
&gt;600 
&gt;400 
&gt;200 
d.sub.50 
No. .mu.m 
.mu.m 
.mu.m 
.mu.m 
.mu.m 
.mu.m 
.mu.m 
.mu.m 
.mu.m 
__________________________________________________________________________ 
1 10.0 
15.0 
27.0 
44.9 
63.8 
81.8 
96.3 
100 950 
2 4.2 10.0 
25.4 
45.5 
63.3 
80.4 
96.3 
100 950 
3 10.9 
15.0 
25.5 
44.1 
63.1 
82.9 
96.7 
99.9 
940 
4 3.7 12.6 
31.1 
52.2 
68.7 
85.0 
97.8 
99.9 
1040 
5 1.9 7.5 
23.8 
42.8 
62.5 
81.1 
96.7 
100 930 
6 3.8 19.0 
38.5 
54.5 
67.5 
82.8 
97.2 
99.9 
1110 
7 13.6 
29.7 
47.8 
52.9 
63.4 
80.8 
97.4 
100 1230 
8 11.9 
18.2 
39.5 
70.9 
89.4 
97.3 
99.8 
100 1270 
9 0 1.2 
8.5 
31.4 
51.2 
59.7 
80.0 
98.9 
810 
10 0 1.6 
6.4 
26.0 
45.1 
54.9 
80.5 
98.4 
720 
11 2.1 6.6 
15.1 
44.9 
58.2 
63.5 
81.5 
97.5 
910 
12 1.7 6.4 
16.6 
46.7 
61.0 
66.8 
82.6 
98.2 
960 
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TABLE 2 
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Cumulative particle size distribution of the ground product 
Example &gt;100 &gt;75 &gt;50 &gt;33 &lt;33 
No. .mu.m .mu.m .mu.m .mu.m 
.mu.m 
______________________________________ 
1 0.2 0.4 4.0 18.0 82.0 
2 0.5 1.0 5.0 21.0 79.0 
3 0 0.2 0.6 1.2 98.8 
4 0 0.2 0.8 1.6 98.4 
5 0 0 0.5 0.8 99.2 
6 0 0 0.1 0.4 99.6 
7 1.0 4.5 11.0 36.0 64.0 
8 1.0 1.8 9.1 30.3 69.7 
9 0 0.5 4.6 14.6 85.4 
10 0 0.2 1.3 7.6 92.4 
11 0.3 2.2 11.4 28.1 71.9 
12 0.2 1.2 7.5 23.1 76.9 
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