Note: Descriptions are shown in the official language in which they were submitted.
wo 96/22315 2 1 9 9 0 1 5 PCTIUSg5116236
TITLE OF THE INVENTION
MICROEMULSION POLYMERIZATION SYSTEMS FOR FLUOROMONOMERS
FIELD OF THE INVENTION
This invention relates to melt-processible fluoropolymers; and to a
process for polymerizing monomers to make the fluoropolymers.
BACKGROUND OF THE INVENTION
Microemulsions are stable isotropic mixtures of oil, water, and surfactant
which form spontaneously upon contact of the ingredients. Other components,
such as salt or co-surfactant (such as an alcohol, amine, or other amphiphilic
molecule) may also be part of the microemulsion formulation. The oil and water
reside in distinct domains separated by an interfacial layer rich in surfactant.Because the domains of oil or water are so small, microemulsions appear
visually Irdnsparent or translucent. Unlike emulsions, microemulsions are
equilibriurn phases.
Microemulsions can have several ",icrosl,uctures, depending mainly
upon composition and sometimes open temperature and pressure. There are
three most common structures. One is an oil-in-water microemulsion in which
oil is contained inside distinct domains in a continuous water-rich domain. The
second is a water-in-oil microemulsion, in which water is contained inside
distinct domains (droplets) in a continuous oil-rich domain. The third is a
bicontinuous microemulsion in which there are sample-spanning intertwined
paths of both oil and water, separated from each other by the surfactant-rich
film.
Polymerization of emulsified and microemulsified unsaturated
hydrocarbon monomers is known, where high reaction rates, high conversions
and high molecular weights can be achieved. A microemulsion can be
distinguished from a conventional emulsion by its optical clarity, low viscosity,
small domain size, thermodynamic stability, and spontaneous formation.
Polymerization of microemulsified monomers has many advantages over
traditional emulsion polymerization. Microemulsions are normally transparent
to translucent so that they are particularly suitable for photochemical reactions,
while emulsions are turbid and opaque. Also, the structural diversity of
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microemulsions (droplets and bicontinuous) is set by thermodynamics, and
rapid polymerization may be able to capture some of the original structure. In
addition, microemulsion polymerization enables production of stable,
monodisperse microlatexes containing colloidal particles smaller than those
produced from classical emulsion polymerization processes. Smaller particle
size improves the ability to form coatings without microcracking. The increased
surface area improves particle fusion during molding operations.
Emulsion polymerization, as opposed to microemulsion polymerization, of
dissolved gaseous tetrafluoroethylene (PTFE) or its copolymers is a known
process. Aqueous colloidal dispersions of PTFE or its copolymers can be
prepared in a pressure reactor by placing the gaseous monomer, or a mixture
of monomers in contact with an aqueous solution containing at least one
surfactant which generally is a fluorinated surfactant, possibly a buffer for
keeping the medium at a given pH, and an initiator which is c~ 'e of forming
free radicals at the polymerization temperature. The free radical initiators canbe water soluble peroxides, or alkaline or ammonium persulfates. Persulfate
can be used alone if the polymerization temperature is above approximately
50C, or in association with a reducing agent such as ferrous salt, silver nitrate,
or sodium bisulfate if the polymerization temperature is approximately between
5 to 55C, as described in the U.S. Patent No. 4,384,092.
The gaseous monomer mo'ecules in the foregoing process enter the
aqueous liquid and react to form polymer without hrst forming a distinct liquid
phase. Thus the polymer pa, licles are large particles suspended in the
aqueous mixture; and the process is not a true liquid-in-liquid emulsion
polymerization. The process is sometimes referred to as dispersion
polymerization.
Additives have been used in atle",pL~ to alter the polymerization
processes and products thereof. For example, in U.S. Patent 3,721,638, a
perfluorinated ether ketone is taught as being added to an aqueous phase
polymerization system for polymerizing tetrafluoroethylene, but the initial
product is in the form of an aqueous gel.
Attempts have been made to prepare tetrafluoroethylene copolymers in
aqueous dispersion systems. For example, EP 0612770 teaches the
copolymerization of TFE and fluoroalkyl perfluorovinyl ethers in an aqueous
system containing methylene chloride to obtain dispersion copolymer particles
of an average of less than 50 nm in size.
U.S. Patent No. 4,864,006 describes the polymerization of TFE and
hexafluoropropylene (HFP) to make a copolymer in an aqueous microemulsion
WO 96t22315 2 1 9 9 0 1 5 PCT/US95/16236
-3-
containing a perfluoropolyether in which the resulting copolymer particles have
a size ranging from 0.041 to 0.070 micrometer.
Microemulsion polymerization operates by a different mechanism than
emulsion polymerization. It involves polymerization of liquid monomer rather
than gaseous monomers. Rec~use the polymerization involves polymerizates
of unusually small cells of liquid monomer, the resulting polymer particles are
unusually small. However, polymerization of liquid TFE is not usually practiced,because of the potential hazards of handling liquid TFE.
It is desirable to provide a process for polymerizing gaseous fluorinated
monomers, such as TFE, to produce homopolymer and copolymer dispersions
in which the particle size of the polymer particles is very small. Microemulsionpolymerization systems would be useful in reaching this goal if a means could
be found for adapting gaseous TFE to polymerization in an aqueous
microemulsion system.
SUMMARY OF THE INVENTION
The aqueous microemulsion polymerization procedure of the invention
involves:
1 ) forming an aqueous microemulsion of a liquid saturated
perfluorinated aliphatic or aromatic hydrocarbon having up to two oxygen,
nitrogen or sulfur atoms and having a mc'ecul~r weight preferably below 500;
2) feeding at least one gaseous polymerizable fluorinated monomer
other than tetrafluoroethylene and, optionally additionally, tetrafluoroethyl~ne to
the microemulsion; and
3) initiating polymerization by adding a free-radical initiator.
The microemulsion is formed by adding the perfluorinated saturated
aliphatic or aromatic hydrocarbon in liquid form and a fluorinated organic
surfactant to water in proportions and at temperatures that result in formation of
a microemulsion.
Very small aqueous dispersion polymer particles are formed as a result of
the polymerization, on the order of 80 nm or less (0.08 micrometer). The
average particle size may be less than 60nm or even less than 30nm. The
polymers produced are thermoplastic, i.e., melt-processible. The polymer is
preferably a copolymer of TFE containing enough comonomer units to render
thermoplastic the typically nonthermoplastic TFE.
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DETAILED DESCRIPTION OF THE INVENTION
In this invention, an aqueous microemulsion polymerization procedure is
described for producing unusually small particles of melt-processible
5 fluoropolymers in which the polymerization is carried out in the presence of
microemulsified seed particles or micelles of a liquid perfluorinated
hydrocarl,on that is a saturated al;phatic or aromatic organic compound having
up to two oxygen, nitrogens, or sulfur atoms and a molecular weight preferably
below 500.
The polymer particles so produced are usually small, being on the order
of one average size of 1 to 80 nanometers (0.001 to 0.080 micrometer,)
preferably 1 to 60 nanometers and most preferably 1 to 30 nanometers. It is
believed that such unusually small polymer particles are obtained because
polymerization of the gaseous TFE takes place inside the very small micelles of
15 the hydrocarbon organic compound in the microemulsion.
The perfluorinated hydrocarbon is a low molecular weight compound that
is liquid at the temperature at which polymerization is carried out. The
molecular weight is preferably less than 500. The perfluorinated hydrocarbon
preferably has a boiling point less than 230C. The perfluorinated hydrocarbon
20 can be a perfluorinated saturated aliphatic compound such as a perfluorinatedalkane; a perfluorinated aromatic compound such as perfluorinated benzene,
or perfluorinated tetradecahydro phenanthene. It can also be a perfluori"ated
alkyl amine such as a perfluorinated trialkyl amine. It can also be a
perfluorinated cyclic aliphatic, such as decalin; and pr~:ferably a heterocyclic25 aliphatic compound containing oxygen or sulfur in the ring, such as perfluoro-2-
butyl tetrahydrofuran.
Examples of perfluorinated hydrocarbons include perfluoro-2-
butyltetrahydrofuran, perfluorodecalin, perfluoromethyldecalin,
perfluorodimethyldecalin, perfluoromethylcyclohexane, perfluoro(1,3-
30 dimethylcyclohexane), perfluorodimethyldecahydronaphthalene,perfluorofluoorene, perfluoro(tetradecahydrophenanthrene),
perfluorotet,dcosane, perfluorokerosenes, octafluoronaphthalene, oligomers of
poly(chlorotrifluoroethylene), perfluoro(trialkylamine) such as
perfluoro(tripropylamine), perfluoro(tributylamine), or perfluoro(tripentylamine),
35 and octafluorotoluene, hexafluorobenzene, and commercial fluorinated
solvents, such as Fluorinert FC-75 produced by 3M. The fluorinated alkanes
can be linear or branched, with a carbon atom number between 3 and 20.
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WO 96/22315 PCT/US95/16236
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Oxygen, nitrogen or sulfur atoms can be present in the molecules, but the
number of such atoms per molecule should be two or less.
The preparation of the microemulsion depends on careful selection of the
ingredients. The microemulsion is prepared by mixing water, perfluorinated
hydrocarbon, fluorinated surfactant(s), and optionally cosolvents or inorganic
salts. The amounts employed are 0.1-40 weight percent, preferably 0.1-20, of
the perfluorinated hydrocarbon;1~0 weight percent, preferably 0.1-25, of the
surfactant and optionally cosu, ractauts; with the remainder water. The
microemulsified perfluorinated hydrocarbons are believed to serve as
microreactors for fluorinated monomers to enter and to be polymerized. The
average particle size of the microemulsions can be in the range of 1 to 80
nanometer, preferably 1 to 60, most preferably 1 to 30. The temperature of the
microemulsion ~ormation can be between 0 to 1 50C, preferably 40 to 100C.
The fluorinated surfactant has the structure Rf E X, where Rf is a
fluorinated alkyl group with a carbon number between 4 and 16, E is an
alkylene group with a carbon number between 0 and 4, and X is an anionic salt
such as COOM, SO3M, SO3NR2, SO4M, a cationic moiety such as
quarternary ammonium salt, or an amphoteric moiety such as aminoxide, or a
non-ionic moiety such as (CH2CH2O)nH; and M is H, Li, Na, K, or NH4; R is a
1 to 5C alkyl group and n is a cardinal number of 2 to 40.
The polymerizable fluorinated monomers that are other than
tetrafluoroethylene, include hexafluoroethylene, perfluoro alkyl vinyl ether,
trifluoroethylene, vinylidene fluoride, vinyl fluoride, chlorotrifluoroethylene.Nonfluori"aled monomers can be used as comonomers, such as vinylidene
chloride, vinyl chloride, ethylene, propylene, butadiene. The monomer is
preferably free-radical polymerizable, and preferably is ethylenically
unsaturated.
To initiate polymerization, the temperature of the microemulsion is
adjusted to between 0 and 150C, preferably 40 to 100C. Initiators for
polymerization include free-radical initiators, such as persulfates, azo initiators,
peroxides, or photo initiators which can generate free radicals by ultraviolet or
gamma rays. Amount of i"ilialors present can range between 0.001 to 5
percent by weight based on the final polymer content. Cosolvents such as an
alcohol, amines or other amphiphilic molecules, or salt can be employed if
desired to facilitate formation of the microemulsion.
The fluorinated gaseous monomers are introduced to the reactor from the
vapor phase into the aqueous microemulsion phase. Sufficient mixing between
liquid and vapor phase is important to encourage mass transfer. The
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mechanism of forming the ultra small fluorinated melt-processible polymer
pa, licles in this invention is not fully understood. It is believed that the higher
the solubility of the monomers in the perfluorinated hydrocarbon, the better to
achieve the original microemulsion particle size and shape. The time of
5 reaction may be between 1 and 500 minutes.
The resulting polymer particles in the resulting dispersion have an
average particle size of between 1 and 80 nanometer, pr~ferdbly 1 to 60, most
preferdbly 1 to 30, and a polymer average molecular weight of over 100,000,
preferably over 1,000,000. The unusually small particle size provides a
10 polymer system with a number of advantages over systems containing larger
particles. The system is an aqueous colloidal dispersion and is clear rather
than turbid.
The small particle size aids in producing coatings of uniform thickness
and aids in imparting good gas permeability of porous subsl,dtes. The
15 fluorinated monomer units in the polymer chain aid in increasing the thermal
stability, hydrophobicity and oleophobicity of subst,dtes to which the polymer is
applied. The polymer so produced can be applied to substrates directly from
the col'oid~' dispersion by immersing the substrate material into the disper~ion,
or by painting the substrate with the dispersion, or by spraying the disper~ion
20 onto the substrate or the like. Suitable subsl, ales include fabrics, woven or
nonwoven materials, screens, papers, or porous or microporous membranes of
any form including sheets or tubes. Once the coating is applied to the
substrate, any water, surfactant or initiators remaining can be drawn off by anyconvenient means, such as heating, steam sl,ipping, vacuum evaporatiorror
25 the like.
The resulting product is a coated substrate with the coating present as a
surface layer if the substrate is non-porous. For porous sub~l,ales, which
include ones made from porous polymer membranes, and especially
microporous polymeric membranes, the coating is ordinarily present as a
30 coating on the internal structure of the substrate that makes up the pores. Aparticularly preferred porous substrate is a microporous polytetrafluoroethylenemade by stretching polytetrafluoroethylene tape or film as described in Gore
U.S. Patent 3,953,566. In this procedure, the structure comprises an
interconnected network of nodes and fibrils interconnecting the nodes, the
35 nodes and fibrils comprising the internal structure that defines the pores. The
resulting coated articles provide gas permeable articles of enhanced
hydrophobicity and oleophobicity and filtration efficiency properties. This
wo 96/22315 2 1 9 9 0 1 5 PCrlUS95/16236
makes them useful as gas filters, vent filters, as insulation for electrical wiring,
and in garment constructions where oil and water repellency is desired.
Test Procedures
Particle Size Determination
A COULTER N4MD particle size analyzer was used. The mean diameter
is measured using light scallering method with helium laser at scattering angle
of 90 degree. Each aqueous dispersion sample was diluted about 10,000
times with deionized water before measurement.
Air Permeability: Gurley Number Test
Gurley numbers were obtained as follows:
The ,~sialdnce of samples to air flow was measured by a Gurley densometer
(ASTM D726-58) manufactured by W. & L. E. Gurley & Sons. The results are
reported in terms of Gurley Number which is the time in seconds for 100 cubic
centimeters of air to pass through 1 square inch of a test sample at a pressure
drop of 4.88 inches of water.
Oil Repellency Test
Oil rating was carried out by MTCC Test Method 118-1983. The higher
the numberl the better the oil repel'en~;y. The highest number is 8.
Meltina Temperature
The melting temperature of a polymer was determined by Dir~zr~nlial
Scan Calorimetric (DSC) analysis at a heating rate of 10C/minute under
nitrogen purge. If it shows a major endull,er,,, at the peak of a certain
temperature, it is reported as the melting temperature of the polymer.
Example 1
In a 2-liter reactor were added 900 grams of deionized water, 50 grams
of Fluorinert FC-75 (obtained from 3M Co., and containing as major ingredient,
perfluoro-2-butyltetrahydrofuran), and 25 grams of ammonium
perfluorooctanoate (Fluororad FC-143, 3M). The mixture formed a transparent
microemulsion phase at room temperature and was stirred at a speed of about
1200 rpm. The reactor was then vacuumed and purged with
tetrafluoroethylene gas three times to ensure oxygen content in the mixture to
be below 30 ppm. Then the temperature of the mixture was raised to and
WO 96/22315 2 1 9 9 0 1 5 PCT/US9S/16236
maintained at about 80C. Then 820 grams of hexafluoropropylene were
charged to the reactor, and the pressure inside the reactor was raised to about
3150 kPa with a supply of tetrafluoroethylene gas. 1.0 gram of ammonium
persulfate initiator in 50 grams of water was pumped into the reactor to start
5 the reaction. The pressure inside the reactor dropped, and was then
maintained at about 3,000 kPa by entering a constant supply of
tetrafluoroethylene gas. The reaction proceeded for about 130 minutes and
was stopped.
The colloidal mixture produced from the above reaction was a clear,
10 transparent dispersion. The solid polymer content was about 6.4% by weight.
The average polymer particle size was about 24 nanometer. Differential Scan
Calorimetric analysis of the polymer shows a major endotherm at the peak of
207C, which is within the typical melting temperature of a copolymer of
tetrafluoroethylene and hexafluoroethylene.
Example 2
In a 2-liter reactor were added 900 grams of deionized water, 25 grams
of Fluorinert FC-75 (3M) (perfluoro-2-butyltetrahydrofuran), and 25 grams of
ammonium perfluorooctanoate (Fluororad FC-143, 3M). The mixture formed a
20 transparent microemulsion phase at room temperature and was stirred at a
speed of about 1200 rpm. The reactor was then vacuumed and purged with
tetrafluoroethylene gas three times to ensure oxygen content in the mixture to
be below 30 ppm. Then the temperature of the mixture was raised to and
maintained at about 80C. 450 grams of hexafluoroproylene was charged to
25 the reactor and the pressure inside the reactor was raised to about 3100 kPa
with a supply of tetrafluoroethylene gas. 1.0 gram of ammonium persulfate in
50 grams of water was pumped into the reactor to start the reaction. The
pressure inside the reactor was maintained at 3100 kPa by addition of a
constant supply of tetrafluoroethylene. The reaction proceeded for about 60
30 minutes after ammonium persulfate was charged. Then an additional 1.5
grams of ammonium persulfate in 50 grams of water was charged to the
reactor. The reaction continued to proceed for another 130 minutes and was
stopped.
The colloidal dispersion produced from the above reaction was a clear,
35 transparent dispersion. The solid polymer content was about 9% by weight.
The average polymer particle size was about 12 nanometer. Differential
Scanning Calorimetric analysis shows a major endotherm at 210C, which is
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within the typical melting temperature range of a copolymer of
tetrafluoroethylene and hexafluoropropylene.
Example 3 - Coated SuL~sl~dles
The dispersions produced from Example 1 and Example 2 were diluted
with 10% by weight ammonium perfluorooctanoate aqueous solution. One part
of dispersion was added with one part of 10% by weight ammonium
perfluorooctanoate aqueous solution. Then the diluted solutions were used to
coat expanded porous polytetrafluoroethylene (PTFE) membranes obtained
from W. L. Gore & Associates. The PTFE membrane had a Gurley number of
10 seconds which means it was air permeable. The dispersions were applied
to the membranes by spraying on one side of the membranes. The
membranes were completely wetted by the dispersion. Excess fluid on the
surface of the membranes was removed by dripping. The coated membranes
were then placed in an oven at 250C for 3 minutes to remove water and
surfactant and then were subject to testing.
Membranes coated with the above described diluted dispersion from
Example 1 and Example 2 had oil repellency rating of 4 and 5 respectively
Gurley numbers between 10 and 15 seconds . The uncoated membranes have
oil repellency rating of only 1.
