Note: Descriptions are shown in the official language in which they were submitted.
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COST-EFFECTIVE ION EXCHANGE MEMBRANES PRODUCED BY A
PHOTOINITIATED POLYMERIZATION PROCESS
TECHNICAL FIELD
[0001] This disclosure relates to ion exchange membranes. More
particularly, this
disclosure relates to producing cost-effective ion exchange membranes using
photoinitiated
polymerization to cure a polymerizable solution comprising: i) a highly
concentrated (> 10 wt %
in the solution) photoinitiator; ii) crosslinker monomer comprising at least
two acrylic groups;
and iii) ionic monomer comprising at least one acrylic group and an ionic
group selected from
one of a sulfonic acid group, a sulfonate group, and a quaternary ammonium
group.
BACKGROUND
[0002] Electrodialysis is a method using a direct electrical current to
transport ions
through ion exchange membranes. Electrodialysis is advantageous and competes
with other
desalination processes, such as reverse osmosis, in many applications.
Lienhard et al (Applied
Energy 2014 136, 649) recently reported that electrodialysis could be a
competitive method to
desalinate high-salinity fracking water. To further expand elelctrodialysis as
a clean treatment
sequence for recovering water in industrial processes, a cost-effective
production of its key
components, ion exchange membranes, is important.
[0003] Based on the fixed ion exchange groups on the membrane matrix, ion
exchange
membrane is categorized into cation exchange membrane (CEM) and anion exchange
membrane
(AEM). Cation exchange membrane (CEM) contains negatively charged groups fixed
to the
polymer matrix and allows the passage of cations but rejects anions, while
anion exchange
membrane (AEM) contains positively charged groups fixed to the polymer matrix
and allows the
passage of anions but rejects cations. After its developments for more than
seventy years, ion
exchange membrane has attained almost an ideal level in the separation between
cations and
anions at any concentration of salt solutions. Ion exchange membrane, however,
has several
technical and economic limitations. A major disadvantage is its high
manufacturing-cost.
Historically, ion exchange membrane is manufactured by a multi-step process
including: 1) first
membrane fabrication through copolymerization of styrene and divinyl benzene
and 2)
subsequent membrane functionalization to introduce ion-exchange moieties onto
the membrane.
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The membrane fabrication and functionalization processes involve use of
hazardous chemicals
exampled by concentrated sulfuric acid or halogenated chemicals and require
elaborate safety
precautions incorporated into the manufacturing facilities and waste stream
handling systems to
mitigate issues associated with worker health issues, and environmental
toxicity.
[0004] US 2011/0097645 describes a process for preparing ion exchange
membrane
using a UV-polymerization. All the examples indicate that membranes are
produced from
curable compositions comprising about 1.0 ¨ 2.0 wt % photoinitiator. With
photoinitiator at this
level, the composition is either cured under an inert atmosphere exampled by
nitrogen or carbon
dioxide, or is cured using a high-energy UV irradiation. The curing rate of a
photoinitiated
polymerization is affected by oxygen inhibition. Oxygen molecules can react
with radicals
produced from photoinitiator and with the propagating polymer radicals,
stopping the
polymerization reaction. An inert atmosphere adds significant costs to the
manufacturing process
while high-energy UV irradiation also adds costs associated with worker health
issues. Similarly
in US 2012/0248032, US 2012/0259027, US 2012/0248028, US 2012/0248029, and US
2012/0248030, all the examples indicate that membranes are produced from
curable
compositions comprising about less than 2.0 wt % photoinitiator.
[0005] There exists a need for a process to produce cost-effective ion
exchange
membranes.
[0006] It is therefore an object of the present invention to produce cost-
effective ion
exchange membranes through a photoinitiated polymerization process.
[0007] It is further an object of the present invention to provide a
polymerizable solution
for producing ion exchange membrane.
[0008] It is further an object of the present invention to provide a
polymerizable solution
that could be cured using UV irradiation in the presence of oxygen.
[0009] It is further an object of the present invention to provide a
process for curing the
polymerizable solution to produce ion exchange membrane wherein the process
could be scaled
up using a roll-to-roll manufacturing process.
SUMMARY
[0010] According to a first aspect, there is provided a polymerizable
solution for
producing an ion exchange membrane. The polymerizable solution comprises: i)
10.0 to 16.0 wt %
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photoinitiator; ii) 20.0 to 50.0 wt % crosslinking monomer comprising at least
two acrylic groups;
iii) 15.0 to 35.0 wt % ionic monomer comprising at least one acrylic group and
an ionic group
selected from one of a sulfonic acid group, a sulfonate group, and a
quaternary ammonium group;
and iv) 14.0 to 45.0 wt % solvent; wherein said acrylic group is acrylate
group or acrylamide
group.
[0011] The crosslinking monomer is selected from one of acrylate-based
crosslinker
comprising at least two acrylate groups and acrylamide-based crosslinker
comprising at least two
acrylamide groups. Suitable crosslinking monomer is exampled by hexanediol
diacrylate,
decanediol diacrylate, polyurethane oligomer diacrylate, polyester oligomer
diacrylate, polyether
oligomer diacrylate, epoxy oligomer diacrylate, polybutadiene oligomer
diacrylate, 1,4-
bis(acryloyl)homopiperazine, hexyl diacrylamide, isophorone diacrylamide, 4,4'-
methylene
bis(phenyl acrylamide), 4,4'-methylene bis(cyclohexyl acrylamide), trimethyl
hexamethylene
diacrylamide, and their mixtures thereof
[0012] The ion exchange membrane might be a cation exchange membrane and
the ionic
monomer comprises at least one acrylic group and an ionic group selected from
one of a sulfonic
acid group and a sulfonate group. Suitable ionic monomer for cation exchange
membrane is
exampled by 3-sulfopropyl acrylate potassium salt, 2-acrylamido-dodecane
sulfonic acid, 2-
acrylamido-2-methyl- 1 -propanesulfonic acid, and their mixtures thereof
[0013] The ion exchange membrane might be an anion exchange membrane and
the ionic
monomer comprises at least one acrylic group and a quaternary ammonium group.
Suitable ionic
monomer for anion exchange membrane is exampled by 3-acrylamidopropyl
trimethylammonium chloride, 2-acryloyloxyethyl trimethylammonium chloride, and
their
mixtures thereof
[0014] According to a second aspect, there is provided a process for
producing an ion
exchange membrane. The process comprises the steps of: 1) preparing a
polymerizable solution
comprising: (i) 10.0 to 16.0 wt % photoinitiator, (ii) 20.0 to 50.0 wt %
crosslinking monomer
comprising at least two acrylic groups, (iii) 15.0 to 35.0 wt % ionic monomer
comprising at least
one acrylic group and an ionic group selected from one of a sulfonic acid
group, a sulfonate
group, and a quaternary ammonium group, and (iv) 14.0 to 45.0 wt % solvent; 2)
applying the
polymerizable solution to a support; and 3) curing the polymerizable solution
using a
photoinitiated polymerization to form an ion exchange membrane.
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[0015] The photoinitiated polymerization is initiated using UV
irradiation.
[0016] The ion exchange membrane is produced using a continuous roll-to-
roll process.
DETAILED DESCRIPTION
[0017] The embodiments of the present disclosure provide a polymerizable
solution for
producing cost-effective ion exchange membranes, in addition to a process for
producing cost-
effective ion exchange membranes through curing the polymerizable solution
using
photoinitiated polymerization, and the membranes produced by the process.
[0018] More specifically, ion exchange membranes in the present
disclosure are
produced using photoinitiated polymerization to cure a polymerizable solution
comprising i) a
highly concentrated (> 10 wt % in the solution) photoinitiator; ii)
crosslinker monomer
comprising at least two acrylic groups; and iii) ionic monomer comprising at
least one acrylic
group and an ionic group selected from one of a sulfonic acid group, a
sulfonate group, and a
quaternary ammonium group. The term "acrylic group" as used herein is acrylate
group or
acrylamide group.
[0019] A first exemplary embodiment of the present disclosure provides a
polymerizable
solution for producing an ion exchange membrane, said polymerizable solution
comprising: (i)
10.0 to 16.0 wt % photoinitiator; (ii) 20.0 to 50.0 wt % crosslinking monomer
comprising at least
two acrylic groups; (iii) 15.0 to 35.0 wt % ionic monomer comprising at least
one acrylic group
and an ionic group selected from one of a sulfonic acid group, a sulfonate
group, and a
quaternary ammonium group; and (iv) 14.0 to 45.0 wt % solvent; wherein said
acrylic group is
acrylate group or acrylamide group.
[0020] Surprisingly, when the photoinitiator concentration in the
polymerizable solution
is at least 10 wt %, ion exchange membrane might be produced by curing the
polymerizable
solution using photoinitiated polymerization in the presence of oxygen,
leading to membrane
production cost-effective. Oxygen inhibition is mainly a surface curing
phenomenon, and
affected by the diffusion of oxygen from the air to the curing coating since a
large excess of
initiator radicals can consume a small amount of oxygen dissolved in coating
solution. Under a
high concentration of photoinitiator, the surface of the coating can be cured
instantly into a thin
polymer film, stopping further oxygen diffusion process. At the same time, the
photoinitiator
concentration also must not be too high to absorb the curing light completely
at the surface,
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ensuring a good through cure of the membrane coating. The preferable
photoinitiator
concentration in the polymerizable solution is about 10 -16 wt %.
[0021] According to one exemplary embodiment, suitable crosslinking
monomer for the
polymerizable solution is exemplified by acrylate-based crosslinker comprising
at least two
acrylate groups, such as hexanediol diacrylate, decanediol diacrylate,
polyurethane oligomer
diacrylate, polyester oligomer diacrylate, polyether oligomer diacrylate,
epoxy oligomer
diacrylate, polybutadiene oligomer diacrylate, and their mixtures thereof.
These acrylate-based
crosslinkers are commercially available from suppliers exemplified by Sartomer
USA LLC
(Exton, PA, USA) and Cytec Industries Inc. (Woodland Park, NJ, USA).
Alternatively, suitable
crosslinking monomer is exemplified by acrylamide-based crosslinker comprising
at least two
acrylamide groups, such as hexyl diacrylamide, 1,4-
bis(acryloyl)homopiperazine, isophorone
diacrylamide 4,4'-methylene bis(phenyl acrylamide), 4,4'-methylene
bis(cyclohexyl acrylamide),
trimethyl hexamethylene diacrylamide, and their mixtures thereof. These
acrylamide-based
crosslinkers might be synthesized by following the methods taught in WO
2010/106356 and WO
2014/059516.
[0022] According to another exemplary embodiment, suitable photoinitiator
for the
polymerizable solution is exampled by a-hydroxy ketones, benzoin ethers,
benzil ketals, a-
dialkoxy acetophenones, a-hydroxy alkylphenones, a-amino alkylphenones,
acylphophine oxides,
benzophenons/amines, thioxanthone/amines, and titanocenes, such as 2-hydroxy-
144-(2-
hydroxyethoxy)phenyl] -2-methyl-l-propanone, 2-hydroxy-2-methy1-1 -phenyl-1 -
propanone, 1 -
hydroxy-cyclohexyl-phenyl-ketone, and mixtures thereof These photoinitiators
are
commercially available from suppliers exemplified by IGM Resins USA
(Charlotte, NC, USA)
and BASF Canada (Toronto, ON, Canada).
[0023] According to another exemplary embodiment, suitable solvent for
the
polymerizable solution is exemplified by diethylene glycol, diethylene glycol
methyl esters, 1,3-
butanediol, N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide,
water, and
mixtures thereof
[0024] The polymerizable solution disclosed herein might be used to
produce cation
exchange membrane wherein the ionic monomer comprising at least one acrylic
group and an
ionic group selected from one of a sulfonic acid group and a sulfonate group.
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[0025] According to another exemplary embodiment, suitable ionic monomer
for the
polymerizable solution to produce cation exchange membrane is exampled by 3-
sulfopropyl
acrylate potassium salt, 2-acrylamido-dodecane sulfonic acid, 2-acrylamido-2-
methyl-1 -
propanesulfonic acid, and their mixtures.
[0026] The polymerizable solution disclosed herein might be used to
produce anion
exchange membrane wherein the ionic monomer comprising at least one acrylic
group and a
quaternary ammonium group.
[0027] According to another exemplary embodiment, suitable ionic monomer
for the
polymerizable solution to produce anion exchange membrane is exampled by 3-
acrylamidopropyl trimethylammonium chloride, 2-acryloyloxyethyl
trimethylammonium
chloride, cationic surfactant monomers described in US Patent Nos. 4,212,820
and 4,918,228,
and their mixtures.
[0028] The polymerizable solution might contain other components exampled
by
viscosity modifier, surfactants and inorganic salts.
[0029] A second exemplary embodiment of the present disclosure provides a
process for
producing an ion exchange membrane comprising the steps of:
1) preparing a polymerizable solution comprising: (i) 10.0 to 16.0 wt %
photoinitiator, (ii)
20.0 to 50.0 wt % crosslinking monomer comprising at least two acrylic groups,
(iii)
15.0 to 35.0 wt % ionic monomer comprising at least one acrylic group and an
ionic
group selected from one of a sulfonic acid group, a sulfonate group, and a
quaternary
ammonium group, and (iv) 14.0 to 45.0 wt % solvent;
2) applying the polymerizable solution to a support; and
3) curing the polymerizable solution using a photoinitiated polymerization to
form an ion
exchange membrane.
[0030] Suitable support is exampled by a woven fabric, a non-woven
fabric, or a
microporous substrate.
[0031] During the applying step, the polymerizable solution might be
applied onto one
side of the flat support forming a layer on top of the support, or might be
applied to both sides of
the flat support forming layers on both sides of the support. The
polymerizable solution might
saturate the support, substantially filling the pores and substantially
covering the surfaces of the
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support. The polymerizable solution might be applied to the support by methods
exemplified by
blade coating, casting, curtain coating, dip-coating, spraying coating and
slot die coating
[0032] During the curing step, curing is performed by irradiating the
solution-applied
support using ultraviolet light with wavelengths of 280 nm ¨ 390 nm. There is
no obvious phase
separation takes place during curing and polymer-formation processes so that
there is no
macropore (pore size of 10 - 1000 nm) structure in the resultant membrane. The
ion exchange
membrane disclosed in the present invention is substantially nonporous. The
resulted ion
exchange membrane has low water permeability so that counterions pass through
the membrane
and water and co-ions might not pass through the membranes.
[0033] In one embodiment, the process disclosed herein might be used to
produce cation
exchange membranes and the ionic monomer comprises at least one acrylic group
and an ionic
group selected from one of a sulfonic acid group and a sulfonate group.
[0034] In another embodiment, the process disclosed herein might be used
to produce
anion exchange membranes and the ionic monomer comprises at least one acrylic
group and a
quaternary ammonium group.
[0035] While it is possible to produce ion exchange membrane using a
batch basis, the
present invention enables the manufacture of ion exchange membranes with a
continuous roll-to-
roll process.
[0036] The ion exchange membranes produced in the present invention
generally have
the following properties: (i) a membrane thickness in the range of about 0.06
mm to about 0.30
mm; (ii) an electrical resistance in the range of about 1.0 Ocm2 to about 8.0
cm2; (iii) a water
content in the range of about 20% to about 45% by weight; and (iv)
permselective for ion pairs
Na/Cl' (0.1M/ 0.5M NaC1 solution) is above 85%.
[0037] The present disclosure will be further illustrated in the
following examples.
However it is to be understood that these examples are for illustrative
purposes only, and should
not be used to limit the scope of the present disclosure in any manner.
EXAMPLES
[0038] It should be noted that in the following Examples, the
permselectivity of ion
exchange membrane was measured using 0.1 and 0.5 M NaC1 solutions at 20 C with
the method
taught by Xu et al. (Fundamental studies of a new series of anion exchange
membranes:
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membrane preparation and characterization. Journal of Membrane Science 2001,
190, 159-166).
The electrical resistance of ion exchange membrane is measured by
electrochemical impedance
spectroscopy in deionized water with the method taught by Lee et al.
(Importance of Proton
Conductivity Measurement in Polymer Electrolyte Membrane for Fuel Cell
Application. Ind. Eng.
Chem. Res. 2005, 44, 7617-7626).
[0039] Chemicals are purchased from Sigma-Aldrich: photoinitiator 2-
hydroxy-2-
methyl-1 -phenyl-propan- 1 -one (HMP), N,N-dimethyl acetamide (DMAc),
diethylene glycol
monobutyl ether (DGME), 1,3-butanediol (Bu-diol), 75 wt% 3-acrylamidopropyl
trimethylammonium chloride solution (APTAC) and 2-acrylamido-2-methyl-1-
propanesulfonic
acid (AMPS). Acrylic crosslinker isophorone diacrylamide (IPDA) was
synthesized as described
in WO 2010/106356. Non-woven polypropylene support PF13040 is provided by
Hollingsworth
& Vose Company (East Walpole, MA, USA).
Examples 1-16
[0040] In a typical procedure, polymerizable solutions for producing ion
exchange
membrane were first prepared by mixing the components as listed in Table 1
(for producing
cation exchange membrane) and in Table 2 (for producing anion exchange
membrane) at room
temperature. The resultant polymerizable solutions were then applied to a non-
woven support
PF13040 using a dip coating process. A membrane was produced through a UV-
initiated
polymerization by travelling the solution-coated support and by irradiating it
with UV light in a
curing station fitted with UV fluorescent light (wavelength 300 nm-400 nm)
bulbs. After curing,
the resultant membranes were washed with 10 wt% NaC1 solution and stored in
0.1 M NaC1
solution.
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Table 1. Polymeriable solution composition for fabricating cation exchange
membrane (CEM),
relative curing time and membrane properties
Component CEM1 CEM2 CEM3 CEM4 CEM5 CEM6 CEM7 CEM8
AMPS (wt %) 26.8 25.8 25.3 24.7 24.2 23.7 23.2
22.6
H20 (wt %) 9.1 8.8 8.6 8.4 8.2 8.0 7.8 7.6
DMAc (wt %) 21.4 20.6 20.2 19.8 19.3 18.9 18.4
18.0
IPDA (wt %) 40.2 38.8 37.9 37.1 36.3 35.4 34.6
33.8
HMP (wt %) 2.5 6.0 8.0 10.0 12.0 14.0 16.0
18.0
*Surface curing (min) 3.5 3.5 2.5 1.5 1.5 1.0 0.5 0.5
*Through curing (min) 5.5 5.5 4.0 2.5 2.0 3.0 4.5 6.0
Resistance (0 cm2) 2.5 2.6 2.6 3.0 3.2 3.5 4.0 5.1
Permselectivity (%) 95 95 93 93 91 90 90 89
*curing time is measured by curing under UV and open to the air 5 ml of
polymerizable solution in a 20
ml transparent scintillation glass vial till the top surface of or the total
solution of the polymerizable
solution becomes firm solid.
Table 2. Polymeriable solution composition (wt %) for fabricating anion
exchange membrane
(AEM), relative curing time and membrane properties
Ingredient AEM1 AEM2 AEM3 AEM4 AEM5 AEM6 AEM7 AEM8
APTAC 28.0 27.0 26.5 25.8 25.3 24.8 24.1 23.6
Bu-diol 8.2 7.9 7.7 7.6 7.4 7.2 7.1 6.9
DGME 5.6 5.4 5.3 5.2 5.0 4.9 4.8 4.7
DMAc 16.7 16.1 15.7 15.4 15.1 14.7 14.4
14.0
IPDA 39.0 37.6 36.8 36.0 35.2 34.4 33.6 32.8
HMP 2.5 6.0 8.0 10.0 12.0 14.0 16.0 18.0
*Surface curing 4.5 4.5 3.0 2.0 1.5 1.0 0.5 0.5
*Through curing 6.5 6.0 4.5 2.5 2.0 3.0 4.5 7.0
Resistance (Q cm2) 2.0 2.1 2.1 2.5 2.8 3.1 3.5 5.0
Permselectivity 92 92 92 90 90 87 86 83
*curing time is measured by curing under UV and open to the air 5 ml of
polymerizable solution in a 20
ml transparent scintillation glass vial till the top surface of or the total
solution of the polymerizable
solution becomes firm solid.
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