Process for the suspension polymerization of tetrafluoroethylene

In the suspension polymerization of tetrafluoroethylene to make granular polymers, presence of a selected salt of a perfluorinated nonanoate enhances desirable properties of the product and the process.

FIELD 
This invention relates to an improvement in the suspension polymerization 
of tetrafluoroethylene to make granular polymer. 
BACKGROUND 
the homopolymerization of tetrafluoroethylene and the copolymerization of 
tetrafluoroethylene with very small amounts of ethylenically unsaturated 
comonomers in aqueous medium can be carried out by two distinctly 
different procedures. In one procedure, called suspension polymerization, 
little or no dispersing agent is employed and agitation is carried out 
vigorously in order to produce a precipitated resin, commonly referred to 
as "granular" resin. In the other procedure, sufficient dispersing agent 
is employed and agitation is mild in order to produce small colloidal size 
particles dispersed in the aqueous reaction medium. In this second 
procedure, called aqueous dispersion polymerization, precipitation (i.e., 
coagulation) of the resin particles is avoided. The two procedures produce 
distinctly different products. The granular product can be molded in 
various forms, whereas the resin produced by the aqueous dispersion method 
cannot be molded but is fabricated by dispersion coating or by converting 
to powder for paste extrusion with a lubricating medium. In contrast, the 
granular resin is incapable of being paste extruded or dispersion coated. 
However, even in the suspension polymerization procedure, presence of small 
amounts of dispersing agent are sometimes beneficial to increase the 
surface area of the granular polymer particles produced. See, for example, 
Anderson, U.S. Pat. No. 3,245,972, when in Example VI, use of 300 ppm 
ammonium perfluorooctanoate, CF.sub.3 --CF.sub.2 --.sub.6 COONH.sub.4, is 
taught. Use of other dispersing agents with additional beneficial 
properties has long been a research goal. 
SUMMARY 
In this invention, a new dispersing agent for use in the suspension 
polymerization of tetrafluoroethylene to make granular polymer is employed 
which surprisingly provides increased surface area in the particles made 
and provides low yield loss and improves polymerization rates. The 
dispersing agent is a perfluorinated nonanoate of the formula 
EQU CF.sub.3 --CF.sub.2 --.sub.7 COOM 
where M is hydrogen, ammonium or an alkali metal. Mixtures of such 
nonanoates may be employed if desired. 
DESCRIPTION OF THE INVENTION 
The polymerization of tetrafluoroethylene, alone or with other 
polymerizable ethylenically unsaturated comonomers, in aqueous medium to 
produce granular tetrafluoroethylene polymer is well known in the art. 
Tetrafluoroethylene monomer, along with ethylenically unsaturated 
comonomer if desired, is typically admixed or contacted with an aqueous 
medium containing a polymerization initiator. Ordinarily, the aqueous 
monomer (or monomers) is introduced into the medium under pressure. 
Typical conditions include polymerization temperatures of 
50.degree.-100.degree. C.; preferably 60.degree.-90.degree. C.; and 
pressures of 10-50.times.10.sup.5 Pa. The polymerization is ordinarily 
carried out in a vigorously stirred autoclave. 
Initiators employed herein are ionic initiators. Examples of ionic 
polymerization initiators include inorganic persulfates such as ammonium 
persulfate or alkali metal persulfates such as potassium persulfate and 
the like. The initiator is added prior to initiation of polymerization. 
The amount of initiator employed depends on the temperature of 
polymerization, the nature of the initiator, the molecular weight of the 
polymer desired, and the rate of reaction desired. Thus, the amount will 
vary depending on the results desired by one skilled in the art. 
Ordinarily, the amount will be between 2 and 500 ppm, based on weight of 
water present. 
A dispersing agent can be employed, if desired, which is an anionic, 
substantially non-telogenic dispersing agent. The amount of dispersing 
agent present, when used, is ordinarily between about 2 ppm and about 200 
ppm based on weight of water employed in the aqueous dispersion and is 
insufficient to cause formation of colloidal polymer particles. The 
dispersing agent is usually added prior to initiation of polymerization. 
If desired, a small amount of a nonionic dispersing agent may be employed 
along with the anionic one. 
Tetrafluoroethylene is either employed alone (to produce the homopolymer, 
polytetrafluoroethylene) or is employed with other copolymerizable, 
ethylenically unsaturated organic comonomer. The amount of comonomer 
employed will depend upon the properties desired in the polymer particles 
obtained, but will not be so great as to result in tetrafluoroethylene 
copolymers that are melt-processible. In other words, the polymers 
obtained are non-melt-processible polymers. Especially preferred 
comonomers, are perfluoro(terminally unsaturated olefins) of 3-7 carbon 
atoms such as hexafluoropropylene, and perfluoro(alkyl vinyl ethers) of 
3-7 carbon atoms such as perfluoro(n-propyl vinyl ether). As used herein 
in both this description and the claims, the term "polytetrafluoroethylene 
means" both homopolymer and copolymers as described herein. 
By the term "non-melt-fabricable", is meant that the polymers have an 
apparent melt viscosity of at least 1.times.10.sup.9 poises at 380.degree. 
C. Melt viscosity is determined by measuring the tensile creep of a 
sintered piece held at 380.degree. C. Specifically, 12 g. of molding 
powder is placed in a 7.6 cm. diameter mold between 0.152 cm. rubber cauls 
and paper spacers. Pressure is then slowly applied on the mold until a 
value of 140.6 kg./cm..sup.2 is obtained. This pressure is held for 2 
minutes and then released slowly. After the sample disc is removed from 
the mold and separated from the cauls and paper spacers, it is sintered at 
380.degree. C. for 30 minutes. The oven is then cooled to 290.degree. C. 
at a rate of about 1.degree. C. a minute and the sample is removed. A 
crack-free rectangular sliver with the following dimensions is cut: 0.152 
to 0.165 cm. wide, 0.152 to 0.165 cm. thick, and at least 6 cm. long. The 
dimensions are measured accurately and the cross-sectional area is 
calculated. The sample sliver is attached at each end to quartz rods by 
wrapping with sliver-coated copper wire. The distance between wrappings is 
4.0 cm. This quartz rod-sample assembly is placed in a columnar oven where 
the 4 cm. test length is brought to a temperature of 
380.degree..+-.2.degree. C. A weight is then attached to the bottom quartz 
rod to give a total weight suspended from the sample sliver of about 4 g. 
The elongation measurements vs. time are obtained, and the best average 
slope for the creep curve in the interval between 30 and 60 minutes is 
measured. The specific melt viscosity, which may be better called apparent 
melt viscosity, is then calculated from the relationship. 
EQU .eta.app=(WL.sub.t g)/3(dL.sub.t /dt)A.sub.T 
where 
.eta.app=(apparent) melt viscosity 
W=tensile load on sample, g 
L.sub.t =length of sample (at 380.degree. C.) cms. (4.32 cm) 
g=gravitational constant, 980 cm./sec..sup.2 
(dL .sub.t /dt)=rate of elongation of sample under load=slope of elongation 
vs. time plot, cm./sec. 
A.sub.T =cross-sectional area of sample (at 380.degree. C.), cm.sup.2 (area 
increases 37% at 380.degree. C. over that at room temperature). 
Polymerization is ordinarily conducted until the solids level (i.e., 
polymer content) of the aqueous mixture is between about 15 and 40 percent 
by weight of the mixture. 
The perfluorinated nonanoate salt used as dispersing agent herein will be 
present in amounts of between 2 ppm and 200 ppm based on weight of water 
used and will be insufficient to cause formation of colloidal particles 
under the suspension polymerization conditions employed. 
The resin produced herein is of the same quality and is useful in the same 
manner as granular tetrafluoroethylene polymers known heretofore. For 
example, the polymers are useful in making molded articles.

The Examples hereinbelow describe the invention in greater detail. Because 
suspension polymerization of tetrafluoroethylene are difficult to 
reproduce exactly due to sensitivity to monomer purity or polymerizer 
cleanliness, polymerization runs carried out on different days are not 
generally comparable. In the following Examples, the designated 
comparisons were carried out on the same day as the Example compared with. 
EXAMPLES 
An autoclave equipped with a two blade agitator was charged with 
demineralized water, "Triton" X-100 solution and the fluorosurfactant in 
the quantities indicated in Table I. In Example 1 and Comparisons A-D, the 
fluorosurfactant was added as a solution in water. The surfactants used in 
B and D were neutralized with ammonium hydroxide. 
The autoclave was heated to the indicated temperature and evacuated and 
purged three times to remove residual air. 
The autoclave was agitated at 600 RPM and a solution of ammonium persulfate 
initiator was added. Tetrafluoroethylene was added until the pressure 
reached 250 psi (1.72M Pa). After the reaction had begun, as evidenced by 
a pressure drop, additional tetrafluoroethylene was added to maintain 250 
psi (1.72M Pa). After the desired amount of tetrafluoroethylene was added, 
the feed was stopped and the monomer allowed to react down until a vacuum 
was obtained. After cooling, the contents of the autoclave were discharged 
and the solids separated from the mother liquor. Any large adhesions which 
formed were removed from the bulk of the polymer. The surface area of the 
dried polymer was determined using the BET method ("Quantasorb"). This 
method is as follows: 
The sample is cooled and nitrogen is absorbed into the polymer. The sample 
is then warmed to desorb the nitrogen which quantity is then measured 
using a thermal conductivity detector. 
The fluorosurfactants used are described as follows: 
FC-143--A mixture of ammonium perfluorocaprylate isomers. The predominate 
isomer is the linear eight carbon chain. 
n-perfluorocaprylic acid 
n-perfluorodecanoic acid 
ammonium omega hydro perfluorononanoate ammonium perfluorononanoate 
TABLE I 
__________________________________________________________________________ 
Example 1 and Comparisons A to D 
A B 1 C D 
__________________________________________________________________________ 
Water, Parts 
227,000 
227,000 
227,000 
227,000 227,000 
"Triton" Solution 
100 100 100 100 100 
(0.226 g/l), 
parts 
Ammonium Persulfate 
1 1 1 1 1 
(parts) 
Fluorosurfactant 
FC-143 Perfluoro 
Ammonium 
Omega Hydro 
Perfluoro 
Type caprylic 
Perfluoro- 
Perfluoro- 
Decanoic 
Acid nanoate 
caprylate 
Acid 
Parts 2.0 1.9 2.2 1.9 2.4 
Moles/Liter 
2.04 .times. 10.sup.-5 
2.04 .times. 10.sup.-5 
2.04 .times. 10.sup.-5 
1.83 .times. 10.sup.-5 
2.04 .times. 10.sup.-5 
Tetrafluoroethylene 
81,720 81,720 81,720 81,720 81,720 
Parts 
Polymerization 
62.degree. C. 
62.degree. C. 
62.degree. C. 
62.degree. C. 
62.degree. C. 
Temperature 
Reaction Time, 
50 63 58 80 82 
Minutes 
Adhesions, Parts 
6810 59,000 18,600 77,180 9080 
% Solids in 
1.30 1.41 1.14 1.91 4.90 
Mother Liquor 
Surface Area, 
3.99 5.02 6.04 4.18 4.55 
m.sup.2 /g 
__________________________________________________________________________ 
The results of these experiments show that: 
i. The dispersing agent of this invention results in the highest surface 
area polymer produced (6.04 m.sup.2 /g). 
ii. The dispersing agent of this invention has the lowest percent solids in 
the mother liquor. Thus it has the lowest yield loss. This is particularly 
valuable in view of the high surface area. Normally to achieve high 
surface area, high dispersing agent concentrations must be used. This 
results in high yield loss to mother liquor. 
iii. A significant difference in reaction time, adhesion level, surface 
area and mother liquor solids exists between Example 1 and Comparison C, 
which differ by only one fluorine atom. 
EXAMPLE 2--GENERAL PROCEDURE 
An autoclave was charged with 69,700 parts demineralized water, one part 
ammonium persulfate, 2.5 parts citric acid (to reduce adhesions), 1.6 
parts of a 0.74% solution of "Triton" X-100 and the indicated 
fluorosurfactant. The autoclave was evacuated to remove air and 
tetrafluoroethylene added to increase the pressure to 250 psi (1.72M Pa). 
The temperature was 65.degree. C. After the reaction began, as evidenced 
by a drop in pressure, additional tetrafluoroethylene was added to 
maintain the pressure at 250 psi (1.72M Pa). After a total of 32,700 parts 
of tetrafluoroethylene was added, the feed was shut off and the autoclave 
allowed to react down to a vacuum. After cooling, the contents were 
discharged. Surface area was determined using a "Quantasorb", as described 
in Example 1. 
TABLE II 
______________________________________ 
Example 2A 
and 2B and Reaction Surface 
Comparisons 
Fluorosurfactant 
Time Area 
E and F Type Parts (Minutes) 
(m.sup.2 gm) 
______________________________________ 
E FC-143 2.7 57.0 3.72 
F FC-143 2.4 54.5 3.51 
2A Ammonium 2.0 49.4 3.90 
Perfluoro- 
nonanoate 
2B Ammonium 1.7 44.6 3.68 
Perfluoro- 
nonanoate 
______________________________________ 
Example 2 illustrates the use of reduced levels of the perfluorononanoate 
in 2A and 2B to achieve a given surface area with a significantly reduced 
reaction time over comparisons E and F.