Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR PREPARING GRAFT COPOLYMER MEMBRANES
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to processes for preparing graft
copolymer membranes by radiation induced graft polymerization of
fluorostyrenic monomers, employing monomer emulsions.
Descr~tion of the Related Art
The preparation of graft polymeric membranes by radiation
induced graft polymerization of a monomer to a polymeric base film has been
demonstrated for various combinations of monomers and base films. The
grafting of styrene to a polymeric base film, and subsequent sulfonation of
the
grafted polystyrene chains has been used to prepare ion-exchange
membranes.
U.S. Patent No. 4,012,303 reports the radiation induced graft
polymerization of a,a,~i-trifluorostyrene (TFS) to polymeric base films using
gamma ray co-irradiation. The graft polymerization procedure may use TFS in
bulk or in solution. The '303 patent reports that aromatic compounds or
halogenated compounds are suitable solvents.
U.S. Patent No. 4,605,685 reports the graft polymerization of TFS
to pre-irradiated polymeric base films. Solid or porous polymeric base films,
such as for example polyethylene and polytetrafluoroethylene, are pre
irradiated and then contacted with TFS neat or dissolved in a solvent.
U.S. Patent No. 6,225,368 reports graft polymerization of
unsaturated monomers to pre-irradiated polymeric base films employing an
emulsion including the monomer, and emulsifier and water. In the method of
the '368 patent, a base polymer is activated by irradiation, quenched so as to
affect cross-linking of the polymer, and then activated again by irradiation.
The
activated, cross-linked polymer is then contacted with the emulsion. The '368
patent also states thafi the use of the disclosed method eliminates
homopolymer'ization caused by irradiation of the monomer, and that this allows
the use of high concentrations of monomers in the emulsion.
These methods of preparing graft polymeric membranes have
several disadvantages.
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With co-irradiation, since the TFS monomer is simultaneously
irradiated, undesirable processes such as monomer dimerization and/or
independent homopolymerization of the monomer may occur in competition
with the desired graft polymerization reaction.
When neat TFS is employed in graft polymerization reactions, it
can be difficult to achieve a contact time between the monomer and the
irradiated base film that would be suitable for high-volume production.
Typically, the neat monomer does not wet the surface of the base film very
effectively, and this can result in an undesirably low graft polymerization
rate
unless a prolonged contact time is employed. Further, the use of neat TFS
may adversely increase the cost of the graft polymerization process, due to
the
excess of monomer that is required.
A disadvantage of graft polymerization reactions carried out using
TFS solutions is the level of graft polymerization drops significantly as the
concentration of monomer in the solution is lowered. Indeed, the '303 patent
reports a significant decrease in percentage graft with decreasing TFS
concentrations. The drop in percentage graft may be mitigated by increasing
the radiation dosage and/or the grafting reaction temperature, but this
necessarily increases the energy requirements of the graft polymerization
process. Overall, the use of TFS in solution tends to undesirably increase the
cost of the graft polymerization process.
Cross-linking the base polymer by irradiating and quenching it
prior to grafting necessitates two separate irradiation steps. Quenching
further
involves heating the irradiated polymer and/or the addition of cross-linking
agents. An obvious disadvantage to this process is that these steps add time
and expense to the process and complicate the overall preparation of the graft
polymeric membranes.
BRIEF SUMMARY OF THE INVENTION
A process for preparing a graft copolymer membrane is provided
comprising exposing a polymeric base film to a dose of ionizing radiation, and
then contacting the irradiated base film with an emulsion comprising a
fluorostyrenic monomer.
In one embodiment, the present process for preparing a graft
copolymer membrane comprises:
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(a) exposing a polymeric base film to a dose of ionizing
radiation; and
(b) contacting the irradiated base film with an emulsion
comprising at least one fluorostyrenic monomer,
wherein the amount of monomer in the emulsion is less than or equal to 30%
by volume.
In another embodiment, the present process for preparing a graft
copolymer membrane comprises exposing a polymeric base film to a dose of
ionizing radiation, and contacting the irradiated base film with an emulsion
comprising at least one substituted a,~i,~-trifluorostyrene monomer.
In another embodiment, the present process for preparing a graft
copolymer membrane comprises exposing a polymeric base film to a dose of
ionizing radiation, and contacting the polymeric base film with an emulsion
comprising trifluoronaphthyl monomers.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic representation of an embodiment of the
present process.
DETAILED DESCRIPTION OF INVENTION
In the present process, a graft copolymer membrane is prepared
by exposing a polymeric base film to a dose of ionizing radiation, and then
contacting the irradiated base film with an emulsion comprising a
fluorostyrenic
monomer.
Any radiation capable of introducing sufficient concentrations of
free radical sites on and within the polymeric base film may be used in the
preparation of the graft copolymer membranes described herein. For example,
the irradiation may be by gamma rays, X-rays, electron beam, or high-energy
UV radiation. The base film may be irradiated in an inert atmosphere. The
radiation dose to which the base film is exposed may vary from 1-100 Mrad.
Typically, the dose range is between 20-60 Mrad.
The polymeric base film may be dense or porous. Typically, the
base film imparts mechanical strength to the membrane and should be
physically and chemically stable to irradiation and the conditions to which it
is to
be exposed in the end-use application of the graft copolymer membrane.
Suitable base films include homopolymers or copolymers of non-fluorinated,
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fluorinated and perfluorinated vinyl monomers. Fluorinated and perfluorinated
polymers may be desired for certain applications due to their enhanced
oxidative and thermal stability. Suitable base films include, but are not
limited
to, films comprising polyethylene, polypropylene, polyvinylidene fluoride,
polytetrafluoroethylene, poly(ethylene-co-tetrafluoroethylene),
poly(tetrafluoroethylene-co-perfluorovinylether), poly(tetrafluoroethylene-co-
hexafluoropropylene), poly(vinylidene fluoride-co-hexafluoropropylene),
poly(vinylidene fluoride-co-chlorotrifluoroethylene), and polyethylene-co-
chlorotrifluoroethylene).
The irradiated base film is then contacted with the emulsion and
monomer is then incorporated into the base film to form a graft copolymer. The
irradiated base film may be contacted with the emulsion in an inert
atmosphere, if desired. The emulsion may assist in wetting the irradiated base
film with the monomer.
Suitable fluorostyrenic monomers include a-fluorostyrenes, a,~3-
difluorostyrenes, a,~,(3-trifluorostyrenes, and the corresponding
fluoronaphthylenes. Unsubstituted and substituted monomers, particularly
para-substituted monomers, may be employed. Mixtures of fluorostyrenic
monomers may also be employed in the emulsion, if desired.
As used herein and in the appended claims, a substituted
fluorostyrenic monomer refers to monomers having substituents on the
aromatic ring. Suitable substituted a,~3,~i-trifluorostyrenes and a,~3,~-
trifluoronaphthylenes are described in PCT Application No. PCT/CA98/01041,
and PCT Application No. PCT/CA00/00337. Examples of such a,~3,~3-
trifluorostyrenes include, but are not limited to, methyl-a,~,~3-
trifluorostyrene,
methoxy-a,~,~-trifluorostyrene, thiomethyl-a,~i,~-trifluorostyrene, and phenyl-
a,~i,~-trifluorostyrene.
The emulsion may further comprise other suitable non-fluorinated
monomers, such as styrene, a-methylstyrene, and vinyl phosphonic acid, for
example. Depending on the end-use application of the graft copolymer
membrane, the incorporation of a proportion of such non-fluorinated monomers
may reduce the cost of the membrane without unduly affecting perfon-nance.
The emulsion may be an aqueous system, i.e., an emulsion
comprising the monomers) and water. Alternatively, a non-aqueous emulsion
may. be employed, comprising the monomers) and an immiscible solvent. l-he
solvent may be selected so as to facilitate swelling of the base film. As a
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further alternative, an aqueous emulsion may be used that also includes a
solvent that facilitates swelling of the base film.
The emulsion may further comprise an emulsifier. Ionic and
nonionic emulsifiers may be employed. Non-limiting examples of suitable
emulsifiers include sodium lauryl sulfate and dodecylamine hydrochloride.
Depending upon the type and concentration of monomers) employed in the
emulsion, an emulsifier may increase the stability of the emulsion. The
particular emulsifier, if it is employed, is not essential and persons skilled
in the
art can readily choose a suitable emulsifier for a given application.
If desired, the emulsion may also comprise an inhibitor to limit the
amount of dimerization and/or homopolymerization of the monomers) that may
occur in the emulsion during graft polymerization. Again, the choice of
inhibitor
is not essential to the present process and suitable inhibitors will be
apparent to
persons skilled in the art.
The graft polymerization reaction may be carried out at any
suitable temperature. Higher temperatures may result in higher graft
polymerization rates, but can also increase the rate of
dimerization/homopolymerization of the monomer. Suitable temperature
ranges will depend on such factors as the desired level of grafting of the
base
film, the graft polymerization rate as a function of fiemperature for the
monomers) employed, and the rate of dimerization/homopolymerization of the
monomers) as a function of temperature. For example, temperatures in the
range of 20-100°C are suitable, with a range of 50-80°C being
typical when
employing a,~3,~i-trifluorostyrenic monomers. Persons skilled in the art can
readily determine suitable temperature ranges for a given application of the
present process.
The method by which the irradiated base film is contacted with
the emulsion is not essential to the present. process. For example, the
irradiated base film may be soaked or dipped in an emulsion bath, or the
emulsion could be coated as a layer onto the irradiated base film.
Alternatively,
the emulsion could be sprayed on, either as an emulsion or as components that
form the emulsion in situ. As a further example, the emulsion could be
contacted with the irradiated base film as a mist. A combination of any of the
foregoing methods may also be employed.
After graft polymerization, the graft copolymer membrane may be
washed in a suitable solvent. The choice of solvent is not essential to the
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present process. Generally, it should be a solvent for the monomer but not for
the base film. Persons skilled in the art can readily determine suitable
solvents
for a particular application.
Ion exchange functionality may then be introduced (directly or
indirectly) into the graft copolymer membrane by subsequent reactions, such
as, halomethylation, sulfonation, phosphonation, amination, carboxylation,
hydroxylation (optionally combined with subsequent phosphorylation) and
nitration, for example, to produce an ion exchange membrane suitable for
various applications. More than one ion exchange moiety may be introduced
into the graft copolymer membrane. Sulfonation or phosphonation, in
particular, may be employed where the graft copolymer membrane is intended
as an ion exchange membrane for use in fuel cell applications.
The particular method of introducing ion exchange functionality
into the grafted film is not essential to the present process, nor is the
selection
of the particular reagent. For example, where a sulfonated graft copolymer
membrane is desired, liquid or vapor phase sulfonation may be employed,
using sulfonating agents such as sulfur trioxide, chlorosulfonic acid (neat or
in
solution), and oleum; with chlorosulfonic acid a subsequent hydrolysis step
may
be required.
Figure 1 is a schematic representation of an embodiment of the
present process. For the purpose of illustration, graft polymerization and
sulfonation of a dense polymeric base film is described. Polymeric base film 2
is fed from roller station 4 to irradiation chamber 6, where it is exposed to
a
dose of ionizing radiation in an inert atmosphere. The irradiated base film
then
moves to grafting chamber 8 where it is exposed to an emulsion comprising a
fluorostyrenic monomer. The monomer is then incorporated into the base film
to form a graft copolymer.
The emulsion is formed in supply 10 and supplied to grafting
chamber 8. Excess emulsion may then be recycled to grafting chamber 8, as
illustrated. As mentioned previously, the monomers) may dimerize instead of
forming a graft copolymer with the irradiated base film. When the
concentration
of dimer in the emulsion increases, the excess emulsion may be directed to
separator 12, which separates monomer and dimer present in the emulsion.
The recycled monomer may 'then be directed to supply 10 for re-use in the
emulsion. Dimer is directed to storage vessel 14. Once a sufficient amount of
dimer is present in storage vessel 14, it is then directed to reactor 16 where
it is
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cracked to form monomer, which may then be directed to supply 10 for re-use
in the emulsion. Such a monomer recovery and recycle system is not required
for the present process, but may increase the efficiency of monomer
utilization
and help to reduce cost.
~ Irradiated base film 2 may exposed to the emulsion by known
methods, as mentioned previously. For example, grafting chamber 8 may
comprise an emulsion bath or a spray booth. Where a porous base film is
employed, it may immersed in an emulsion bath to imbibe the emulsion into the
interior, and then sprayed with the emulsion, as well.
Where an emulsion bath is employed, means for agitating the
emulsion may be employed, if desired. Conventional means for agitating the
emulsion include stirring, sparging and ultrasonicating. Agitating may assist
in
maintaining the homogeneity of the emulsion.
Graft copolymer membrane 2 is then supplied to wash station 18
where it is washed in a suitable solvent. Solvent is provided to wash station
18
from solvent supply 20. Waste material may be separated from the solvent in
separator 22 and the solvent recycled, as illustrated. Graft copolymer
membrane 2 is then supplied to sulfonation chamber 24 and sulfonated therein.
As with the monomer recovery and recycle system discussed
above, the illustrated solvent recycle system described in Figure 1 is not
required for the present process, but may increase the efficiency of solvent
utilization in the graft polymerization process and help to reduce the cost
and/or
reduce the environmental impact of process waste streams.
In the illustrated embodiment, graft copolymer membrane 2 is
sulfonated by sulfur trioxide vapor. If desired, graft copolymer membrane 2
could be sulfonated at elevated pressure and/or temperature to enhance the
rate of sulfonation. The sulfur trioxide may be diluted with an inert gas,
such as
nitrogen, to reduce its activity, as well. If desired, graft copolymer
membrane 2
could be pre-soaked in a solvent to swell it, thereby facilitating sulfonation
of
the inferior of the membrane. Suitable solvents include halogenated solvents
such as 1,2-dichloroethane and 1,1,2,2-tetrachloroethane, for example.
However, other sulfonation reagents and/or conditions may be employed in the
present process, as discussed above.
Of course, other ion exchange functionality could be introduced
into graft copolymer membrane 2, such as those discussed above.
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Sulfonated membrane 2 is then directed to water wash station 26.
The wash water is recovered and recycled, and waste is collected in vessel 28
for disposal, as illustrated.
.. Sulfonated membrane 2 is then dried in station 30 before being'
collected at roller station 32.
EXAMPLE 1
EMULSION GRAFT POLYMERIZATION OF PARA-METHYL
-a,(3,(3-TRIFLUOROSTYRENE (P-ME-TFS) TO
POL.YVINYLIDENE FLUORIDE (TEDLAR~ SP) FILM
Four samples of 25 ~,m thick polyvinylidene fluoride (TedIar~R' SP)
film (7 cm x 7 cm) were irradiated with a dose of 20 Mrad using a 10 MeV ion
beam radiation source, in an inert atmosphere with dry ice cooling. A 30%
(v/v)
emulsion was prepared by adding neat, degassed p-Me-TFS and
dodecylamine hydrochloride to water (DDA.HCI; 0.050 g/ml water). Two
irradiated base film samples were then immersed in the emulsion at 80°C
for 2-
3 hours, in an inert atmosphere. The other two samples were exposed to neat,
degassed p-Me-TFS under the same reaction conditions. The p-Me-TFS
grafted films were then washed twice with acetone and once with toluene
before being dried at 45°C in a vacuum (3.9 kPa) for 3 hours. The
percentage
graft polymerization for each sample was then determined by calculating the
percentage increase in mass of the grafted film relative to the mass of the
base
film.
The reaction conditions and percentage graft polymerization for
each sample is summarized in Table 1.
Table 1
Emulsion graft polymerization of p-Me-TFS
to of v_irwlidene fluoride film
Sample Emulsion Time % Craft
or Neat (h)
1 neat 2 71.8
2 emulsion 2 93.5
3 neat 3 77.6
4 emulsion 3 104.2
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EXAMPLE 2
EMULSION GRAFT POLYMERIZATION OF P-ME-TFS TO
POLY(ETHYLENE-CO-CHLOROTRIFLUOROETHYLENE) (HALAR~) FILM
7 cm x 7 cm samples of poly(ethylene-co-chlorotrifluoroethylene)
(Halar~) film were prepared from 25 ~,m and 50 ~m thick Halar~ 300LC and
Halar~ MBF (porous film; 630 p,m thick, 204 g/m2). The samples were irradiated
with a dose of 10-40 Mrad using a 10 MeV ion beam radiation source.
Samples 5-20 were irradiated in an inert atmosphere with dry ice cooling. An
emulsion was prepared as described in Example 1. Half the samples were
then immersed in the emulsion at 60-80°C for 24 hours, in an inert
atmosphere.
The remaining half of the samples were exposed to neat, degassed p-Me-TFS
under the same reaction conditions. The p-Me-TFS grafted films were then
washed twice with acetone and once with toluene before being dried at
45°C in
a vacuum (3.9 kPa) for 3 hours. The percentage graft polymerization for each
sample was then determined as described in Example 1.
The reaction conditions and percentage graft polymerization for
each sample is summarized in Table 2.
Table 2
Emulsion~raft polxmerization of,a-Me-TFS to
polyethylene-co-chlorotrifluoroethylene film
Dense ThicknessDose EmulsionTemperatureoho Craft
Sample or (p,m) (Mrad) or Neat (C)
porous
5 dense 25 10 neat 60 48.0
6 dense 25 10 emulsion60 80.8
7 dense 25 20 neat 60 58.5
8 dense 25 20 emulsion60 88.6
9 dense 25 40 neat 60 62.1
10 dense 25 40 emulsion60 105.5
11 dense 25 10 neat 70 44.9
12 dense 25 10 emulsion70 64.9
13 dense 25 20 neat 70 50.6
14 dense 25 20 emulsion70 88.7
15 dense 25 10 neat 80 19.C
16 dense 25 10 emulsio n- 80 56.~
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Dense or ThicknessDose EmulsionTemperatureoho
Sample porous (p,m) (Mrad) or Neat (C) Graft
17 dense 50 20 neat 60 56.0
18 dense 50 20 emulsion60 98.2
19 porous 630 20 neat 60 40.8
20 porous 630 ~ 20 emulsion60 68.9
EXAMPLE 3
EMULSION GRAFT POLYMERIZATION OF P-ME-TFS TO
POLY(ETHYLENE-CO-TETRAFLUOROETHYLENE) (TEFZEL~) FILM
Samples of 2 mil (approximately 50 Vim) thick polyethylene-co-
tetrafluoroethylene) (Tefzel~) film (7 cm x 7 cm) were irradiated with a dose
of
20 Mrad using a 10 MeV ion beam radiation source, in an inert atmosphere
with dry ice cooling. Emulsions were prepared by adding neat, degassed
p-Me-TFS and dodecylamine hydrochloride (DDA.HCI) to water at varying
concentrations. Two irradiated base film samples were then immersed in a
given emulsion at 80°C for 2 hours, in an inert atmosphere. In
addition, sample
53 was exposed to neat, degassed p-Me-TFS under the same reaction
conditions. The p-Me-TFS grafted films were then washed twice with acetone
and once with toluene before being dried at 45°C in a vacuum (3.9 kPa)
for 3
hours. The percentage graft polymerization for each sample was then
determined as described in Example 1.
The reaction conditions, emulsion composition and percentage
graft polymerization for each sample tested is summarized in Table 3.
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Table 3
Emulsion graft polymerization of p-Me-TFS to
poly(ethylene-co-tetrafluoroethylene) film
DDA. HCI Averag
Sampl % Monomer concentratio% Graft a
a (by weight) n (g/ml Graft
water)
21 10 0.006 34.5
22 10 0.006. 32.9 33.7
23 30 0.006 47.7
24 30 0.006 48.7 48.2
25 50 0.006 50.8
26 50 0.006 49.7 50.2
27 70 0.006 51.5
28 70 0.006 51.3 51.4
29 10 0.050 59.8
30 10 0.050 62.2 61.0
31 30 0.050 59.9
32 30 0.050 57.3 58.6
33 50 0.050 56.1
34 50 0.050 56.5 56.3
35 70 0.050 52.5
36 70 0.050 51.8 52.2
37 10 0.100 63.0
38 10 0.100 62.7 62.9
39 30 0.100 58.5
40 30 0.100 57.9 58.2
41 50 0.100 55.7
42 50 0.100 54.8 55.3
43 70 0.100 51.3
44 70 0.100 51.1 51.2
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DDA.HCI Averag
Sampl % Monomer concentratio% Graft a
a (by weight) n (g/ml Graft
water)
45 10 0.170 58.7
46 10 0.170 54.1 56.4
47 30 0.170 54.0
48 30 0.170 55.4 54.7
49 50 0.170 53.1
50 50 0.170 52.7 52.9
51 70 0.170 49.4
52 70 0.170 48.6 49.0
53 100 _ 36.2 36.2
(neat)
As shown in Tables 1-3, with the exception of samples 21 and 22,
the emulsion graft polymerized samples exhibited higher graft polymerization
rates relative to the comparative examples using the neat monomer under
otherwise identical reaction conditions. Thus, the present process can achieve
higher graft polymerization rates using less monomer than can be achieved
when employing neat monomer under similar reaction conditions.
Also note that, with the exception of samples 21-28, there is a
trend of increasing percentage graft polymerization with lower concentration
of
the monomer in the emulsion (see Table 4). Indeed, the highest percentage
grafts for samples 29-52 were achieved using an emulsion having 10%
monomer. This result for the emulsion graft polymerization process is
surprising, since graft polymerization using TFS in solution yields lower
percentage grafts with decreasing monomer concentration.
EXAMPLE 4
SULFONATION OF POLY(ETHYLENE-CO-TETRAFLUOROETHYLENE)-G-P-ME-TFS
Samples 29 and 30 of Example 3 were sulfonated as follows.
Each sample was immersed in a sulfonation solution (30% S03 in
dichloroethane, with 5% w/w acetic acid) for 2 hr at 50 °C. The EW of
tt-~e
sulfonated samples was determined, as was the amount of water present in the
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samples. From this data the percentage sulfonation of the samples was
determined. Percentage sulfonation is measured as the percentage of
available sites on the graft copolymer that are sulfonated, assuming one
sulfonate group per site. The sulfonation results are summarized in Table 4.
Table 4
Sulfonation of poly~ethLrlene-co-tetrafluoroethylene)-a-p-Me-TFS
Water
Sampl % Graft EW (g/mol) content (wt.Sulfonation
a
29 59.8 591 35.3 94.1
30 62.2 574 37.7 95.1
The present process provides for the preparation of graft
copolymer membranes from fluorostyrenic monomers that is straightforward
and makes efficient use of the monomers. The ability to use lower
concentrations of monomer than is currently employed in solution or emulsion
graft polymerization of fluorostyrenic monomers, while achieved comparable or
superior graft polymerization rates, allows for considerable cost savings in
membrane production, particularly in high-volume, continuous production.
All of the above U.S. patents, U.S. patent application publications,
U.S. patent applications, foreign patents, foreign patent applications and non-
patent publications referred to in this specification and/or listed in the
Application Data Sheet, are incorporated herein by reference in their
entirety.
While particular elements, embodiments and applicafiions of the
present invention have been shown and described, it will be understood, of
course, that the invention is not limited thereto since modifications may be
made by those skilled in the art, particularly in light of the foregoing
teachings.
It is therefore contemplated by the appended claims to cover such
modifications that incorporate those features coming within the scope of the
invention.
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