U.S. patent application number 11/293778 was filed with the patent office on 2006-06-22 for viscosity control for reduced fluorosurfactant aqueous fluoropolymer dispersions by the addition of cationic surfactant.
Invention is credited to Robert John Cavanaugh.
Application Number | 20060135681 11/293778 |
Document ID | / |
Family ID | 36192118 |
Filed Date | 2006-06-22 |
United States Patent
Application |
20060135681 |
Kind Code |
A1 |
Cavanaugh; Robert John |
June 22, 2006 |
Viscosity control for reduced fluorosurfactant aqueous
fluoropolymer dispersions by the addition of cationic
surfactant
Abstract
A stabilized aqueous fluoropolymer dispersion comprising
fluoropolymer particles, wherein the dispersion contains less that
about 300 ppm fluorosurfactant based on the weight of the
dispersion and a cationic surfactant. The cationic surfactant aids
in reducing the viscosity of the dispersion and preferred cationic
surfactants are volatile in coating or film casting operations
involving drying and sintering of the dispersion.
Inventors: |
Cavanaugh; Robert John;
(Hilton Head Island, SC) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY;LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1128
4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
36192118 |
Appl. No.: |
11/293778 |
Filed: |
December 2, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60638310 |
Dec 22, 2004 |
|
|
|
Current U.S.
Class: |
524/544 |
Current CPC
Class: |
C08F 6/16 20130101; C08F
6/16 20130101; C08L 27/12 20130101; C08L 27/18 20130101; C08K 5/19
20130101; C08K 5/19 20130101; C08L 27/12 20130101; C09D 127/18
20130101; C08K 5/19 20130101 |
Class at
Publication: |
524/544 |
International
Class: |
C08L 27/12 20060101
C08L027/12 |
Claims
1. A stabilized aqueous fluoropolymer dispersion comprising
fluoropolymer particles, said dispersion containing less that about
300 ppm fluorosurfactant based on the weight of said dispersion,
said dispersion comprising a cationic surfactant.
2. The dispersion of claim 1 wherein said fluoropolymer particles
have an average particle size or about 100 to about 350 nm.
3. The dispersion of claim 1 wherein the solids content of the
dispersion is about 30 to about 70 wt %.
4. The dispersion of claim 1 wherein the solids content of the
dispersion is about 45 to about 70 wt %.
5. The dispersion of claim 1 wherein said cationic surfactant is
present in an amount of at least about 100 ppm based the weight of
the dispersion.
6. The dispersion of claim 1 wherein said cationic surfactant is
present in an amount of about 100 ppm to about 1000 ppm based on
the weight of the dispersion.
7. The dispersion of claim 1 further comprising non-ionic
surfactant.
8. The dispersion of claim 7 wherein said non-ionic surfactant is
present in an amount of about 1 to about 5 wt % based the weight of
the dispersion.
9. The dispersion of claim 1 wherein said cationic surfactant is
quaternary ammonium halide or hydroxide.
10. The dispersion of claim 8 wherein said cationic surfactant is a
compound of the formula: ##STR2## wherein R.sub.1 is a long chain
alkyl hydrocarbon, an alkylated aryl hydrocarbon and R.sub.2,
R.sub.3 and R.sub.4 are the same or different and are alkyl groups,
the alkyl groups may contain cyclic structures and may be either
linear or branched, and Y can be F, Cl, Br or OH.
11. The dispersion of claim 10 wherein R.sub.1 is an alkyl group
containing between 8 and 28 carbon atoms.
12. The dispersion of claim 10 wherein R.sub.1 is an alkyl group
containing between 12 and 18 carbon atoms.
13. The dispersion of claim 10 wherein R.sub.2, R.sub.3 and R.sub.4
are the same or different and are selected from the group
comprising alkyl groups having 1-16 carbon atoms.
14. The dispersion of claim 10 wherein R.sub.2, R.sub.3 and R.sub.4
are the same or different and are selected from the group
comprising methyl or ethyl.
15. The dispersion of claim 1 wherein said dispersion contains less
than about 100 ppm fluorosurfactant based on the weight of said
dispersion.
16. The dispersion of claim 1 wherein said dispersion contains less
than about 50 ppm fluorosurfactant based on the weight of said
dispersion.
17. A process for reducing fluorosurfactant content of a stabilized
fluorosurfactant-containing aqueous fluoropolymer dispersion
comprising: reducing the fluorosurfactant content of said
stabilized fluorosurfactant-containing aqueous fluoropolymer
dispersion to a predetermined level; and adding cationic surfactant
to said dispersion.
18. The process of claim 17 wherein said stabilized
fluorosurfactant-containing aqueous fluoropolymer dispersion has a
solids content of about 15 to about 70 wt %.
19. The process of claim 17 wherein said cationic surfactant is
added in an amount of at least about 100 ppm based the weight of
the dispersion.
20. The process of claim 17 wherein said cationic surfactant is
added in an amount of at about 100 ppm to about 1000 ppm based the
weight of the dispersion.
21. The process of claim 17 wherein said stabilized
fluorosurfactant-containing aqueous fluoropolymer dispersion
comprises nonionic surfactant.
22. The process of claim 21 wherein said non-ionic surfactant is
present in an amount of about 1 to about 5% based the weight of the
dispersion.
23. The process of claim 21 further comprising concentrating said
dispersion.
24. The process of claim 21 wherein said reducing of
fluorosurfactant content is performed prior to said
concentrating.
25. The process of claim 21 wherein said cationic surfactant is
added prior to said concentrating.
26. The process of claim 17 wherein said stabilized dispersion has
a solids content of at least about 25 wt %.
27. The process of claim 17 wherein said stabilized dispersion has
a solids content of at least about 30 wt %.
28. The process of claim 17 wherein said stabilized dispersion has
a solids content of at least about 35 wt %.
29. A process for reducing fluorosurfactant content of a stabilized
fluorosurfactant-containing aqueous fluoropolymer dispersion
comprising: contacting said stabilized fluorosurfactant-containing
aqueous fluoropolymer dispersion with an anion exchange resin to
reduce fluorosurfactant content to a predetermined level;
separating said anion exchange resin from said dispersion after the
fluorosurfactant content has been reduced; and adding cationic
surfactant to said dispersion.
30. The process of claim 29 wherein said anion exchange resin has
functional groups selected from the group comprising primary amine,
secondary amine, tertiary amine, and quaternary ammonium
groups.
31. The process of claim 29 wherein said anion exchange resin
comprises quaternary ammonium groups.
32. The process of claim 29 wherein said anion exchange resin is in
hydroxide form.
33. The process of claim 29 wherein said contacting said stabilized
fluorosurfactant-containing aqueous fluoropolymer dispersion with
anion exchange resin reduces fluorosurfactant content to a
predetermined level of less than about 300 ppm.
34. The process of claim 29 wherein said contacting said stabilized
fluorosurfactant-containing aqueous fluoropolymer dispersion with
anion exchange resin reduces fluorosurfactant content to a
predetermined level of less than about 100 ppm.
35. The process of claim 29 wherein said contacting said stabilized
fluorosurfactant-containing aqueous fluoropolymer dispersion with
anion exchange resin reduces fluorosurfactant content to a
predetermined level of less than about 50 ppm.
36. A process for controlling the viscosity of a stabilized reduced
fluorosurfactant aqueous fluoropolymer dispersion comprising:
adding a cationic surfactant in an amount sufficient to reduce
dispersion viscosity to a predetermined level.
Description
FIELD OF THE INVENTION
[0001] This invention relates to removing fluorosurfactant from
aqueous fluoropolymer dispersions and more particularly relates to
controlling of the viscosity of the resulting dispersions.
BACKGROUND OF THE INVENTION
[0002] Aqueous fluoropolymer dispersions are typically manufactured
in an aqueous dispersion polymerization employing anion
fluorosurfactants as a polymerization aid, the fluorosurfactants
functioning as a non-telogenic dispersing agent. For example, an
early description of this use of anionic fluorosurfactants is found
in U.S. Pat. No. 2,559,752 (Berry). After polymerization, the
dispersions are usually subjected to a concentration step to
increase the fluoropolymer solids concentration in the dispersion.
In one type of dispersion concentration operation, the dispersion
is concentrated with the aid of a nonionic surfactant as taught in
Marks et al., U.S. Pat. No. 3,037,953, and in Holmes, U.S. Pat. No.
3,704,272 to raise the solids from nominally 35 wt % in the raw
dispersion to about 60 wt % in the concentrated dispersion. Miura
et al., U.S. Pat. No. 6,153,688 discloses a similar process.
Anionic fluorosurfactants and nonionic surfactants are usually
present in the concentrated dispersions.
[0003] Because of environmental concerns and because anionic
fluorosurfactants are expensive, processes have been developed for
the removal of anionic fluorosurfactants from aqueous fluoropolymer
dispersions. One method for removal of anionic fluorosurfactants
from fluoropolymer dispersions is disclosed in U.S. Pat. No.
4,369,266 and includes the addition of a stabilizing surfactant
followed by concentration by ultrafiltration. This patent teaches
that a high proportion of the fluorosurfactant can be removed via
the aqueous permeate. It is also known to remove anionic
fluorosurfactant by adsorption onto an ion exchange resin as taught
in U.S. Pat. No. 3,882,153 (Seki et al) and U.S. Pat. No. 4,282,162
(Kuhls). Kuhls teaches recovery of fluorinated emulsifiers
dissolved in the aqueous phase after coagulation of the polymer
from the dispersion or in aqueous polymer dispersions to be
concentrated. In anion exchange processes, the anionic
fluorosurfactant is removed by the anion exchange resin from a
stabilized dispersion containing nonionic surfactant.
[0004] In concentrated aqueous fluoropolymer dispersions which have
reduced levels of anionic fluorosurfactant, the viscosity levels
can be higher than in dispersions containing fluorosurfactant and
can be unacceptably high for some end uses. Certain types of
fluoropolymer dispersion, particularly high molecular weight
polytetrafluoroethylene dispersions, show an increase to an
unusually high viscosity when the anionic fluorosurfactant content
is significantly reduced. Viscosity can rise to a level of several
hundred centipoise (cP), well above the normal 20-30 cP which is
advantageous for coating and impregnating compositions and to make
cast films. U.S. Pat. No. 2004/0186219 A1 and U.S. Pat. No.
2003/0171736 A1 (Dadelas et al.) teach the addition of a
non-fluorinated anionic surfactant, e.g., sodium lauryl sulfate,
sodium dodecylbenzyl sulphonate and secondary alkyl sulphonate
sodium salt, to reduce viscosity. However, such non-fluorinated
anionic surfactants introduce ionic species into the dispersion
which are not volatile under the conditions used for drying and
sintering the fluoropolymer in coating or film casting operations.
The presence of residues from such species can produce
objectionable color in films and coatings and may give rise to long
term stability problems.
BRIEF SUMMARY OF THE INVENTION
[0005] The present invention provides a stabilized aqueous
fluoropolymer dispersion comprising fluoropolymer particles,
wherein the dispersion contains less that about 300 ppm
fluorosurfactant based on the weight of the dispersion and a
cationic surfactant. The cationic surfactant aids in reducing the
viscosity of the dispersion and preferred cationic surfactants,
unlike the anionic surfactants used by Dadelas et al., are
completely volatilized in coating or film casting operations
involving drying and sintering of the dispersion.
[0006] The invention further provides a process for reducing the
viscosity of a stabilized fluorosurfactant-containing aqueous
fluoropolymer dispersion by reducing the fluorosurfactant content
of the stabilized fluorosurfactant-containing aqueous fluoropolymer
dispersion to a predetermined level and adding cationic surfactant.
In a preferred embodiment the fluorosurfactant content of a
stabilized fluorosurfactant-containing aqueous fluoropolymer
dispersion is reduced by contacting the stabilized
fluorosurfactant-containing aqueous fluoropolymer dispersion with
an anion exchange resin.
DETAILED DESCRIPTION OF THE INVENTION
Fluoropolymer Dispersion
[0007] The stabilized fluorosurfactant-containing aqueous
fluoropolymer dispersion for treatment in accordance with the
present invention is made by dispersion polymerization (also known
as emulsion polymerization). The aqueous fluoropolymer dispersion
is a stabilized fluorosurfactant-containing aqueous fluoropolymer
dispersion which means that it contains sufficient nonionic
surfactant to prevent coagulation of the dispersion when the
fluorosurfactant content is reduced. As will be explained in more
detail hereinafter, depending upon when the process of the
invention is employed, the nonionic surfactant may already be
present or may be added for stabilization prior to treatment
according to the invention. After concentration, aqueous
fluoropolymer dispersions are useful in coating or impregnating
compositions and to make cast films.
[0008] Fluoropolymer dispersions are comprised of particles of
polymers made from monomers wherein at least one of the monomers
contains fluorine. The fluoropolymer of the particles of the
aqueous dispersions used in this invention is independently
selected from the group of polymers and copolymers of
trifluoroethylene, hexafluoropropylene,
monochlorotrifluoroethylene, dichlorodifluoroethylene,
tetrafluoroethylene, perfluoroalkyl ethylene monomers,
perfluoro(alkyl vinyl ether) monomers, vinylidene fluoride, and
vinyl fluoride.
[0009] The invention is especially useful when the fluoropolymer
component of the dispersion is polytetrafluoroethylene (PTFE)
including modified PTFE which is not melt-processible.
Polytetrafluoroethylene (PTFE) refers to the polymerized
tetrafluoroethylene by itself without any significant comonomer
present. Modified PTFE refers to copolymers of TFE with such small
concentrations of comonomer that the melting point of the resultant
polymer is not substantially reduced below that of PTFE. The
concentration of such comonomer is preferably less than 1 weight %,
more preferably less than 0.5 weight %. The modified PTFE contains
a small amount of comonomer modifier which improves film forming
capability during baking (fusing), such as perfluoroolefin, notably
hexafluoropropylene (HFP) or perfluoro(alkyl vinyl) ether (PAVE),
where the alkyl group contains 1 to 5 carbon atoms, with
perfluoro(ethyl vinyl) ether (PEVE) and perfluoro(propyl vinyl)
ether (PPVE) being preferred. Chlorotrifluoroethylene (CTFE),
perfluorobutyl ethylene (PFBE), or other monomer that introduces
bulky side groups into the molecule are also included. The PTFE
typically has a melt creep viscosity of at least 1.times.10.sup.9
Pas. Such high melt viscosity indicates that the PTFE does not flow
in the molten state and therefore is not melt-processible. PTFE and
modified PTFE are frequently sold in dispersion form and
transported in shipping containers and the process of the invention
can be readily employed for reducing the fluorosurfactant content
of such dispersions.
[0010] The fluoropolymer component of the dispersion may be
melt-processible. By melt-processible, it is meant that the polymer
can be processed in the molten state (i.e., fabricated from the
melt into shaped articles such as films, fibers, and tubes etc.
that exhibit sufficient strength and toughness to be useful for
their intended purpose). Examples of such melt-processible
fluoropolymers include copolymers of tetrafluoroethylene (TFE) and
at least one fluorinated copolymerizable monomer (comonomer)
present in the polymer in sufficient amount to reduce the melting
point of the copolymer substantially below that of TFE homopolymer,
polytetrafluoroethylene (PTFE), e.g., to a melting temperature no
greater than 315.degree. C. Such fluoropolymers include
polychlorotrifluoroethylene, copolymers of tetrafluoroethylene
(TFE) or chlorotrifluoroethylene (CTFE). Preferred comonomers with
of TFE are perfluoroolefin having 3 to 8 carbon atoms, such as
hexafluoropropylene (HFP), and/or perfluoro(alkyl vinyl ether)
(PAVE) in which the linear or branched alkyl group contains 1 to 5
carbon atoms. Preferred PAVE monomers are those in which the alkyl
group contains 1, 2, 3 or 4 carbon atoms, and the copolymer can be
made using several PAVE monomers. Preferred TFE copolymers include
FEP (TFE/HFP copolymer), PFA (TFE/PAVE copolymer), TFE/HFP/PAVE
wherein PAVE is PEVE and/or PPVE and MFA (TFE/PMVE/PAVE wherein the
alkyl group of PAVE has at least two carbon atoms). The
melt-processible copolymer is made by incorporating an amount of
comonomer into the copolymer in order to provide a copolymer which
typically has a melt flow rate of about 1-100 g/10 min as measured
according to ASTM D-1238 at the temperature which is standard for
the specific copolymer. Typically, the melt viscosity will range
from 10.sup.2 Pas to about 10.sup.6 Pas, preferably 10.sup.3 to
about 10 .sup.5 Pas measured at 372.degree. C. by the method of
ASTM D-1238 modified as described in U.S. Pat. No. 4,380,618.
Additional melt-processible fluoropolymers are the copolymers of
ethylene or propylene with TFE or CTFE, notably ETFE, ECTFE and
PCTFE. Further useful polymers are film forming polymers of
polyvinylidene fluoride(PVDF) and copolymers of vinylidene fluoride
as well as polyvinyl fluoride (PVF) and copolymers of vinyl
fluoride.
[0011] A typical process for the aqueous dispersion polymerization
of preferred polymer PTFE is a process wherein TFE vapor is fed to
a heated reactor containing fluorosurfactants, paraffin wax and
deionized water. If desired, a chain transfer agent can also be
employed to reduce the molecular weight to the desired level. A
free-radical initiator solution is added and, as the polymerization
proceeds, additional TFE is added to maintain the pressure. The
exothermic heat of reaction is removed by circulating cooling water
through the reactor jacket. After several hours, the feeds are
stopped, the reactor is vented and purged with nitrogen, and the
raw dispersion in the vessel is transferred to a cooling vessel.
Paraffin wax is removed and polymer dispersion is transferred to a
dispersion concentration operation which produces stabilized
dispersions which are useful for the practice of the present
invention. In the dispersion concentration operation, the
dispersion is concentrated with the aid of a nonionic surfactant as
taught in Marks et al., U.S. Pat. No. 3,037,953, and in Holmes,
U.S. Pat. No. 3,704,272 to raise the solids from nominally 35 wt %
to about 60 wt %. Miura et al., U.S. Pat. No. 6,153,688 discloses a
similar process. Aromatic alcohol ethoxylates can be used as the
nonionic surfactant but, because there is concern about possible
environmental harm from aromatic compounds, preferred nonionic
surfactants are aliphatic alcohol ethoxylates. Suitable nonionic
surfactants include any of a variety of aliphatic alcohol
ethoxylates or mixtures thereof which provide the desired cloud
point during concentration and which provide desired properties in
the dispersion, e.g., low burn off temperature, dispersion
stability, etc. Especially preferred nonionic surfactants are a
compound or mixture of compounds of the formula:
R(OCH.sub.2CH.sub.2).sub.nOH wherein R is a branched alkyl,
branched alkenyl, cycloalkyl, or cycloalkenyl hydrocarbon group
having 8-18 carbon atoms and n is an average value of 5 to 18.
[0012] Concentrated dispersions containing nonionic surfactant made
as described above thus are stabilized fluorosurfactant-containing
dispersions suitable for use in the practice of the present
invention.
[0013] The dispersion polymerization of melt-processible TFE
copolymers is similar except that one or more comonomers are added
to the batch initially and/or introduced during polymerization. In
addition, a telogen such as a hydrocarbon is employed to control
the molecular weight to achieve the desired melt flow of the
polymer for the intended purpose. The same dispersion concentration
operation performed with the aid a nonionic surfactant as used for
PTFE dispersions can be used for TFE copolymer dispersions.
[0014] Stabilized fluorosurfactant-containing dispersions suitable
for use in the practice of the present invention can be made prior
to concentration by adding nonionic surfactants to raw dispersion
(after wax removal referred to above). It is desirable to use the
same nonionic surfactants that will be used for concentration and
nonionic surfactants for this purpose are described above. Nonionic
surfactant is typically added to the raw dispersion under mild
agitation conditions in approximately the same concentrations as is
used for concentration, about 2 to about 6% based on the weight of
the dispersion.
[0015] Stabilized fluorosurfactant-containing fluoropolymer
dispersions with solids content of about 15 to about 70 wt % are
advantageously treated in accordance with the invention.
Preferably, the solids content is at least about about 25 wt %,
more preferably at least about 30 wt %, and most preferably at
least about 35 wt %.
Fluorosurfactants
[0016] The anionic fluorosurfactant in the
fluorosurfactant-containing dispersions to be reduced in this
process is a non-telogenic, anionic dispersing agent, soluble in
water and comprising an anionic hydrophilic group and a hydrophobic
portion. Preferably, the hydrophobic portion is an aliphatic
fluoroalkyl group containing at least four carbon atoms, all except
at most one of which, and that one the closest to the solubilizing
group, bearing at least two fluorine atoms, the terminal carbon
atom bearing in addition an atom consisting of hydrogen or
fluorine. These fluorosurfactants are used as a polymerization aid
for dispersing and, because they do not chain transfer, they do not
cause formation of polymer with undesirable short chain length. An
extensive list of suitable fluorosurfactants is disclosed in U.S.
Pat. No. 2,559,752 to Berry. Preferably, the fluorosurfactant is a
perfluorinated carboxylic acid having 6-10 carbon atoms and is
typically used in salt form. Suitable fluorosurfactants are
ammonium perfluorocarboxylates, e.g., ammonium perfluorocaprylate
or ammonium perfluorooctanoate. The fluorosurfactants are usually
present in the amount of 0.02 to 1 wt % with respect to the amount
of polymer formed.
Cationic Surfactants
[0017] The cationic surfactant used in accordance with the
preferred embodiment of the present invention is a quaternary
ammonium halide or hydroxide, preferably a compound of the formula:
##STR1## where R.sub.1 is a long chain alkyl hydrocarbon, an
alkylated aryl hydrocarbon and R.sub.2, R.sub.3, and R.sub.4 are
alkyl groups, preferably having 1-16 carbon atoms. The alkyl groups
may contain cyclic structures and may be either linear or branched
and Y can be F, Cl, Br or OH. Preferably R.sub.1 is an alkyl group
containing between 8 and 28 carbon atoms, more preferably between
12 and 18 carbon atoms.
[0018] In one preferred embodiment, R.sub.2, R.sub.3, and R.sub.4
are the same or different and are selected from the group
comprising methyl or ethyl. Two especially effective surfactants
are dodecyl trimethyl ammonium bromide and cetyl trimethyl ammonium
bromide (CTMAB). CTMAB is also known as hexadecyl trimethyl
ammonium bromide.
[0019] The cationic surfactants preferably used in accordance with
the invention preferably have the advantage of being completely
volatile at PTFE sintering temperatures. Therefore, they do not
introduce residues into sintered PTFE when the dispersions in
accordance with the invention are applied in coating and film
applications.
[0020] The amount of the cationic surfactant sufficient to reduce
dispersion viscosity to a predetermined desirable level depends on
a number of factors including the nature of the polymer, the amount
of residual fluorosurfactant if present, the solids content of the
dispersion and the amount and type of non-ionic surfactant present.
Determining an optimum level of cationic surfactant for commercial
production can easily be ascertained empirically by one skilled in
the art using samples of the dispersion. In some cases, a small
amount of a cationic surfactant may initially increase the
viscosity of the dispersion before enough is added to reduce the
viscosity. Typically, the viscosity is reduced significantly by the
addition of amounts of the cationic surfactant in slight excess
over the small amount which produces a viscosity increase but
larger amounts do not provide further significant decrease in
viscosity. In a preferred embodiment of this invention, the
cationic surfactant is added in an amount of at least about 100
ppm, preferably about 100 ppm to about 1000 ppm based on the weight
of the dispersion. Preferably, the amount of cationic surfactant
added reduces the viscosity of the concentrated dispersion to less
than about 50 cP, more preferably less than about 40 cP, and most
preferably less than about 30 cP.
Anion Exchange Resins
[0021] The anion exchange resins for use in accordance with the
preferred embodiment of the present invention can be either
strongly basic or weakly basic. Suitable weakly basic anion
exchange resins contain primary, secondary amine, or tertiary amine
groups. Suitable strongly basic anion exchange resin contain
quaternary ammonium groups. Although weakly basic resins are useful
because they can be regenerated more easily, strongly basis resins
are preferred when it is desired to reduce fluorosurfactant to very
low levels and for high utilization of the resin. Strongly basic
ion exchange resins also have the advantage of less sensitivity to
the pH of the media. Strongly basic cation exchange resins have an
associated counter ion and are typically available in chloride or
hydroxyl ion form but are readily converted to other forms if
desired. Anion exchange resins with hydroxyl, chloride, sulfate,
and nitrate can be used for the removal of the fluorosurfactant but
anion exchange resins in the form of a hydroxyl counter ion are
preferred to prevent the introduction of additional anions and to
increase pH during anion exchange because a high pH, i.e., greater
than 9, is desirable in the product prior to shipping to inhibit
bacterial growth. Examples of suitable commercially-available
strong base anion exchange resins with quaternary ammonium groups
with a trimethylamine moiety include DOWEX.RTM. 550A, US Filter
A464-OH, SYBRON M-500-OH, SYBRON ASB1-OH, PUROLITE A-500-OH, ltochu
TSA 1200, AMBERLITE.RTM. IR 402. Examples of suitable
commercially-available stong base anion exchange resins with
quaternary ammonium groups with a dimethyl ethanol amine moiety
include US Filter A244-OH, AMBERLITE.RTM. 410, DOWEX.RTM. MARATHON
A2, and DOWEX.RTM. UPCORE Mono A2.
[0022] Anion exchange resin preferably used in the process of the
present invention is monodisperse. More preferably, the anion
exchange resin beads have a number average size distribution in
which 95% of the beads have a diameter within plus or minus 100
.mu.m of the number average bead diameter.
[0023] The monodisperse anion exchange resin has a particle size
which provides a suitable pressure drop through the bed. Very large
beads are fragile and prone to breakage. Very small ion exchange
beads are susceptible to tight particle packing resulting in
tortuous channels in the bed. This can result in high shear
conditions in the bed. Preferred ion exchange resin has a number
average bead size about 450 to about 800 .mu.m, more preferably,
the ion exchange resin beads have a number average bead diameter of
about 550 to about 700 .mu.m.
Process
[0024] The present invention permits reducing the fluorosurfactant
content of a fluorosurfactant-containing dispersion to a
predetermined level, preferably a level no greater than about 300
ppm, more preferably a predetermined level no greater than about
100 ppm, especially a predetermined level no greater than about 50
ppm.
[0025] The fluorosurfactant content can be reduced by any of the
procedures as have been described by the prior art. One method for
removal of fluorosurfactants from fluoropolymer dispersions is
disclosed in U.S. Pat. No. 4,369,266 and includes the addition of a
stabilizing surfactant followed by concentration by
ultrafiltration. In the preferred embodiment of the present
invention, the fluorosurfactant is removed by adsorption onto an
ion exchange resin as taught has been taught in U.S. Pat. No.
3,882,153 (Seki et al) and U.S. Pat. No. 4,282,162 (Kuhls).
[0026] Contacting of the anion exchange resin with the dispersion
can occur before or after concentration but typically the lower
solids material before concentration is easier to process,
especially when a fixed bed is employed for carrying out the
contacting step. If the process is carried out prior to
concentration, nonionic surfactants are added prior to contact with
the anion exchange resin as discussed above.
[0027] Any of a variety of techniques which bring the dispersion in
contact with the anion exchange resin can be used to carrying out
ion exchange of the process. For example, the process can be
carried out by addition of ion exchange resin bead to the
dispersion in a stirred tank followed by separation of dispersion
from the anion exchange resin beads by filtration. Another suitable
method is to pass the dispersion through a fixed bed of anion
exchange resin instead of using a stirred tank. Flow can be upward
or downward through the bed and no separate separation step is
needed since the resin remains in the fixed bed.
[0028] The contacting of the dispersion is performed at a
temperature which is sufficiently high to facilitate the rate of
ion exchange and to reduce the viscosity of the dispersion but
being below a temperature at which the anion exchange resin
degrades at a detrimentally high rate. The process should be run at
a temperature below the cloud point of the non-ionic surfactant to
prevent phase separation during the ion exchange process. Upper
treatment temperature will vary with the type of polymer employed.
Typically, temperatures will be between 20.degree. C. and
80.degree. C. Preferably, the temperature is between about
45.degree. C. and 65.degree. C., more preferably between about
50.degree. C. and 60.degree. C.
[0029] The fluorosurfactant can be recovered from the anion
exchange resin if desired or the resin with the fluorosurfactant
can be disposed of in an environmentally acceptable method, e.g.,
by incineration. If it is desired to recover the fluorosurfactant,
the fluorosurfactant may be removed from resin by elution. Elution
of fluorosurfactant adsorbed on the anion exchange resin is readily
achieved when weak base resins are used by use of ammonia solution
as demonstrated by Seki in U.S. Pat. No. 3,882,153, by a mixture of
dilute mineral acid with organic solvent (e.g., HCl/ethanol) as
demonstrated by Kuhls in U.S. Pat. No. 4,282,162, or by strong
mineral acids such as sulfuric acid and nitric, transferring the
adsorbed fluorinated carboxylic acid to the eluent. The
fluorosurfactant in the eluent in high concentration can easily be
recovered in the form of a pure acid or in the form of salts by
common methods such as acid-deposition, salting out, or other forms
of concentration, etc.
[0030] The cationic surfactant can be added to the stabilized
dispersion at various times as the process is carried out.
Preferably, the cationic surfactant is added after removal of the
fluorosurfactant because the cationic surfactant may form a complex
with the fluorosurfactant, hampering its removal. The cationic
surfactant can be added after concentration if desired. However,
the significant increase in viscosity which is observed in some
reduced fluorosurfactant dispersions upon concentration can be
avoided if the cationic surfactant is added prior to
concentration.
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