THIN FILM COMPOSITE MEMBRANES INCORPORATING CARBON NANOTUBES
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] For the United States of America, this is an application claiming the benefit of
co-pending U.S. Application Serial Nos. 12/895,353 and 12/895,365 filed on
September 30, 2010, all of which are incorporated herein by reference.
BACKGROUND
[0002] Reverse osmosis (RO) desalination uses membrane technology to transform
seawater and brackish water into fresh water for drinking, irrigation and industrial
applications. Reverse osmosis desalination processes require substantially less
energy than thermal desalination processes. As a result, the majority of recent
commercial projects use more cost-effective reverse osmosis membranes to produce
fresh water from seawater or brackish water. Over the years, advances in
membrane technology and energy recovery devices have made reverse osmosis
more affordable and efficient. Despite its capacity to efficiently remove ionic species
at as high as99.8% salt rejection, there remains a need for reverse osmosis
membranes that possess improved flux characteristics while maintaining useful
rejection characteristics.
[0003] Reverse osmosis is the process of forcing a solvent from a region of high
solute concentration through a membrane to a region of low solute concentration by
applying a pressure in excess of the osmotic pressure. This is the reverse of the
normal osmosis process, which is the natural movement of solvent from an area of
low solute concentration, through a membrane, to an area of high solute
concentration when no external pressure is applied. The membrane here is
semipermeable, meaning it allows the passage of solvent but not of solute. The
membranes used for reverse osmosis have a dense barrier layer where most
separation occurs. In most cases the membrane is designed to allow only water to
pass through this dense layer while preventing the passage of solutes (such as salt
ions). Examples of reverse osmosis processes are the purification of brackish water
and seawater, where often less than 1% of the impurity species in the seawater or
brackish water are found in the permeate. The reverse osmosis process requires that
a high pressure be exerted on the high concentration side of the membrane, usually
2-1 7 bar (30-250 psi) for fresh and brackish water, and 40-70 bar (600-1000 psi)
for seawater, which has around 24 bar (350 psi) natural osmotic pressure which must
be overcome.
[0004] Nanofiltration, in concept and operation, is much the same as reverse
osmosis. The key difference is the degree of removal of monovalent ions such as
chlorides. Reverse osmosis removes about 99% of the monovalent ions.
Nanofiltration membranes removal of monovalent ions varies between 50% to 90%
depending on the material and manufacture of the membrane. Nanofiltration
membranes and systems are used for water softening, food and pharmaceutical
applications. An example of a nanofiltration process is the desalting of a sugar
solution, where 80% of the salt passes through the membrane with the water and
95% of the sugar is retained by the membrane.
[0005] It is well known that for a given polymer, there is a flux-rejection trade-off
curve that defines the upper bound of the flux-rejection relationship. One can obtain
high membrane flux with trade-off in terms of salt rejection. On the other hand, one
can obtain high membrane salt rejection with trade-off in terms of membrane water
permeability. It is highly desirable to obtain membrane materials with performance
above the trade-off curve, i.e., achieving both high flux and high salt rejection.
[0006] Nanotubes such as carbon and boron nanotubes are fundamentally new
nanoporous materials that have great potential for membrane applications. The
current methods of synthesis of CNT membranes (Hinds et al Science, 2004; Holt et.
al. Science, 2006; Fornasiero et. al., PNAS, 2008) involve multiple steps and are
limited to making membrane samples of extremely small area. They are not scalable
to large surface areas necessary for the fabrication of commercial membranes for
practical applications. Membranes containing carbon nanotubes have been
disclosed for use in purifying water. For example, WO 2006/060721 , assigned to
National University of Singapore, describes thin film composite (TFC) membranes
containing multi-walled carbon nanotubes (MWNT) in an active layer prepared by
interfacial polymerization. The WNTs are characterized as having an outside
diameter of 30-50 nm. However, further improvements in the performance of TFC
membranes for reverse osmosis applications are desirable.
BRIEF DESCRIPTION
[0007] It has been unexpectedly discovered that incorporating multi-walled carbon
nanotubes having an outside diameter of less than about 30 n in a thin film
composite membrane by an interfacial polymerization process may yield an
improvement in properties of the membrane.
DETAILED DESCRIPTION
[0008] In a first aspect, the present invention relates to processes for manufacturing
a thin film composite membrane comprising multi-walled carbon nantubes. The
processes include contacting under interfacial polymerization conditions an organic
solution comprising a polyacid halide with an aqueous solution comprising a
polyamine to form a thin film composite membrane on a surface of a porous base
membrane; at least one of the organic solution and the aqueous solution further
including multi-walled carbon nanotubes having an outside diameter of less than
about 30 nm.
[0009] In another aspect, solvents for use in compositions containing nanotubes may
have density greater than about 0.8 and solubility in water of less than about 100 g L.
The solvent may be a single compound or a mixture having the specified density and
water solubility. Particularly suitable solvents having these properties are cis- and
frans-decalin, and mixtures thereof. Solvents for use in the processes of the present
invention may additionally be insoluble in the polysulfone base membranes
commonly used in preparing reverse osmosis membranes. The terms "polysulfone
insoluble" and "insoluble in polysulfones" means that such materials swell or dissolve
polysulfone. These materials typically contain one or more double or triple bonds, for
example, C=C, C=0, and S=0. Examples include cyclohexanone, N-methyl
pyrrolidone (NMP), dimethyl acetate (DMAc), dimethyl sulfoxide (DMSO) and
sulfolane. Polysulfone insoluble materials may be included in compositions of the
present invention in minor amounts, that is, less than about 50% by weight, based on
total weight of the composition. In some embodiments, the polysulfone insoluble
materials are present at less than or equal to about 10% by weight, in others at less
than or equal to about 5% by weight, and in still others, less than or equal to about
3% by weight.
[0010] Other properties of solvents that may be relevant to nanotube dispersion
and/or suitability for use in interfacial polymerization include stability, including
viscosity, and boiling point. In some cases, higher viscosity may yield more stable
dispersions. It may be desirable for solvents that must be removed from a polymer
formed from the compositions of the present invention that the boiling point be
relatively low, typically lower that about 200°deg C.
[001 1] In some embodiments, the solvent is a saturated cyclic C -C2 hydrocarbon
solvent. In particular embodiments, the saturated cyclic C5-C2o hydrocarbon solvent
is a saturated polycyclic compound, or a mixture or one or more saturated polycyclic
compounds, for example, cis-decalin, trans-decalin, cyclohexyl halides, and 1,5,9-
cyclododecatrien and derivatives or a mixture thereof.
[001 2] Compositions of the present invention and organic solutions for use in the
processes of the present invention may also include at least one saturated acyclic
C4-C30 alkane compound, such as hexane or one or more isoparaffins. Suitable
isoparaffins include the ISOPAR™ series from ExxonMobil (including, but not limited
to, ISOPAR™ E, ISOPAR™ G, ISOPAR™ H, ISOPAR™ L, and ISOPAR™ M). The
saturated cyclic C 5-C20 hydrocarbon loading in the organic solution is greater than
about 20% w w (weight of saturated cyclic C 5-C20 hydrocarbon/total weight of solvent,
not including monomer or nanotubes); in some embodiments, greater than about
50% w/w, and in other embodiments, greater than about 80% w/w.
[001 3] The organic solution may additionally include a cyclic ketone such as
cyclooctanone, cycloheptanone, 2 methylcyclohexanone, cyclohexanone,
cyclohexene-3-one, cyclopentanone, cyclobutanone, 3-ketotetrahydrofuran, 3-
ketotetrahydrothiophene, or 3-ketoxetane, particularly, cyclohexanone. Aqueous
dispersions may include dispersing aids such as polyvinylpyrrolidone, or surfactants,
particularly non-ionic surfactants.
[0014] Compositions of the present invention and organic solutions for use in the
processes of the present invention may also include other additives. The additive
loading in the solvent mixture is in the range of 0.1 to 20 wt%, preferably in the range
of 0.5% to 10%, and more preferably in the range of 1 to 10%. These other
additives include the following compounds with molar volumes in the range of 50
cm3/mol-1 or higher (preferably 80 or higher) and Hildebrand solubility parameters in
the range of 8.5 to 10.5 cal cm . aromatic hydrocarbons such as tetralin,
dodecylbenzene, octadecylbenzene, benzene, toluene, xylene, mesitylene, anisole,
dimethylbenzenes, trimethylbenzenes, tetramethylbenzene, ethyl-benzene
.fluorobenzene, chlorobenzene, bromobenzenes, dibromobenzenes, iodobenzene,
nitrobenzene, ethyl-toluene, pentamethyl-benzene, octyl-benzene, cumene, pseudocumene,
para-cymene, phenetole, and phenoxy-decane; naphthalenes such as
methylnaphtha!enes, dimethylnaphthalenes, trimethylnaphthalenes,
ethylnaphthalenes phenylnaphthalenes, chloronaphthalenes, dichloronaphthalenes,
bromonaphthalenes, dibromonaphthalenes nitronaphthalenes, and dinitropyrenes;
ketones such as cyclopentanone, cyclohexanone, and alkylcyclohexanones; and
conjugated oligomers, polymers, and copolymers, including poly(m-phenylene
vinylene), poly(p-phenylene vinylene), poly(3-alkylthiophene), and poly(arylene
ethynylene).
[001 5] Nanotubes for use with solvents having density greater than about 0.8 and
solubility in water of less than about 100 g L include single wall, double wall, and
multiwall carbon nanotubes and boron nitride nanotubes with various internal and
external diameters and length. Nanotubes with carboxyl (COOH), hydroxyl (OH)
carbonyl chloride (-COCI), functionalized with octadecylamine, functionalized with
PEG (polyethylene glycol) may be used. Nanotubes with carbonyl chloride (-COCI)
may be covalently bonded to polyamide thin film to avoid the leach out of nanotubes
during membrane service. The nanotubes typically have a cylindrical nanostructure
with an inside diameter (ID) and outside diameters (OD). Concentration of the
nanotubes in the organic solution or the aqueous solution is at least 0.025% w/w, and
may range from about 0.025% w/w to about 10% w/w in some embodiments, and in
others, ranges from about 0.025% w/w to about 5% w/w. In yet other embodiments,
the concentration of the nanotubes ranges from about 0.05% w/w to about 1% w/w.
[0016] The thin film composite (TFC) membranes that may be prepared by a process
according to the present invention are composed of a separating functional layer
formed on a porous base support. The separating functional layer is thin in order to
maximize membrane flux performance, and is formed on a porous support or base
membrane to provide mechanical strength. Examples of TFC membranes that may
be prepared include, but are not limited to, reverse osmosis membranes composed
of a polyamide separating functional layer formed on a porous polysulfone support,
nanofiltration membranes, and other thin film composite membrane.
[001 7] Interfacial polymerization includes contacting an aqueous solution of one or
more nucleophilic monomers onto a porous support membrane; followed by coating
an organic solution, generally in an aliphatic solvent, containing one or more
electrophilic monomers. At the interface of the two solution layers, which lies near
the surface of the porous support, a thin film polymer is formed from condensation of
the electrophilic and nucleophilic monomers and is adherent to the porous support.
The rate of thin film formation may be accelerated by heating or addition of catalysts.
The polyacid halide monomer on contact with the polyamine monomer reacts on the
surface of the porous base membrane to afford a polyamide disposed on the surface
of the porous support membrane. Suitable monomers useful in the present invention
are described below.
[0018] As described above, the membrane comprises a polymer having an amine
group. The polymer may be produced by interfacial polymerization. Interfacial
polymerization includes a process widely used for the synthesis of thin film
membranes for reverse osmosis, hyperfiltration, and nanofiltration. Interfacial
polymerization includes coating a first solution, generally aqueous, of one or more
nucleophilic monomers onto a porous base support; followed by coating a second
solution, generally in an aliphatic solvent, containing one or more electrophilic
monomers. The second solution is immiscible with the first solution. At the interface
of the two solution layers, which lies near the surface of the porous base support, a
thin film polymer is formed from condensation of the electrophilic a d nucleophilic
monomers and is adherent to the porous base support. The rate of thin film formation
may be accelerated by heating or addition of catalysts.
[0019] Examples of nucleophilic monomers include, but are not limited to, amine
containing monomers such as polyethylenimines; cyclohexanediamines; 1,2-
diaminocyclohexane; 1,4-diaminocyclohexane; piperazine; methyl piperazine;
dimethylpiperazine (e.g. 2,5-dimethyl piperazine); homopiperazine; 1,3-
bis(piperidyl)propane; 4-aminomethylpiperazine; cyclohexanetriamines (e.g. 1,3,5-
triaminocyclohexane); xylylenediamines (o-, m-, p-xylenediamine);
phenylenediamines; {e.g. m-phenylene diamine and p-phenylenediamine, 3,5-
diaminobenzoic acid, 3,5-diamonsulfonic acid); chloropheny!enediamines (e.g. 4- or
5-chloro-m-phenylenediamine); benzenetriamines (e.g. 1,3,5-benzenetriamine, 1,2,4-
triaminobenzene); bis(aminobenzyl) aniline; tetraaminobenzenes; diaminobiphenyls
(e.g. 4,4,'-diaminobiphenyl; tetrakis(aminomethyl)methane;
diaminodiphenylmethanes; N,N'-diphenylethylenediamine; aminobenzamides (e.g. 4-
aminobenzamide, 3,3'-diaminobenzamide; 3,5-diaminobenzamide; 3,5-
diaminobenzamide; 3,3'5,5'-tetraaminobenzamide); either individually or in any
combinations thereof.
[0020] Particularly useful nucleophilic monomers for the present invention include
m-phenylenediamine, p-phenylenediamine, 1,3,5-triaminobenzene, piperazine, 4-
aminomethylpiperidine, and either individually or in any combinations thereof. More
particularly, nucleophilic monomer useful in the present invention includes mphenylene
diamine.
[0021] Examples of electrophilic monomers include, but are not limited to, acid
halide-terminated polyamide oligomers (e.g. copolymers of piperazine with an excess
of isophthaloyl chloride); benzene dicarboxylic acid halides (e.g. isophthaloyl chloride
or terephthaloyl chloride); benzene tricarboxylic acid halides (e.g. trimesoyl chloride
or trimellitic acid trichloride); cyclohexane dicarboxylic acid halides (e.g. 1,3-
cyclohexane dicarboxylic acid chloride or 1,4-cyclohexane dicarboxylic acid chloride);
cyclohexane tricarboxylic acid halides (e.g.cis-1 ,3,5-cyclohexane tricarboxylic acid
trichloride); pyridine dicarboxylic acid halides (e.g. quinolinic acid dichloride or
dipicolinic acid dichloride); trimellitic anhydride acid halides; benzene tetra carboxylic
acid halides (e.g. pyromellitic acid tetrachloride); pyromellitic acid dianhydride;
pyridine tricarboxylic acid halides; sebacic acid halides; azelaic acid halides; adipic
acid halides; dodecanedioic acid halides; toluene diisocyanate; methylenebis(phenyl
isocyanates); naphthalene diisocyanates; bitolyl diisocyanates; hexamethylene
diisocyanate; phenylene diisocyanates; isocyanato benzene dicarboxylic acid halides
(e.g. 5-isocyanato isophthaloyl chloride); haloformyloxy benzene dicarboxylic acid
halides (e.g. 5-chloroformyloxy isophthaloyl chloride); dihalosulfonyl benzenes (e.g.
1,3-benzenedisulfonic acid chloride); halosulfonyl benzene dicarboxylic acid halides
(e.g. 3-chlorosulfonyl isophthaloyl chloride); 1,3,6-tri(chlorosulfonyl)naphthalene;
1,3,7 tri(chlorosulfonyl)napthalene; trihalosulfonyl benzenes (e.g. 1,3,5-
trichlorosulfonyl benzene); and cyclopentanetetracarboxylic acid halides, either
individually or in any combinations thereof.
[0022] Particular electrophilic monomers include, but are not limited to, terephthaloyl
chloride, isophthaloyl chloride, 5-isocyanato isophthaloyl chloride, 5-chloroformyloxy
isophthaloyl chloride, 5-chlorosulfonyl isophthaloyl chloride, 1,3,6-
(trichlorosutfonyl)naphthalene, 1,3,7-(trichlorosulfonyl)napthalene, 1,3,5-
trichlorosulfonyl benzene, either individually or in any combinations thereof. More
particular electrophilic monomers include trimesoyl chloride acid chloride
[0023] The interfacial polymerization reaction may be carried out at a temperature
ranging from about 5°C to about 60°C, preferably from about 10°C to about 40°C to
produce an interfacial polymer membrane. Examples of interfacial polymers
produced thereform include polyamide, polysulfonamide, polyurethane, polyurea, and
polyesteramides, either individually or in any combinations thereof.
[0024] In one example, for illustration and not limitation, a porous base support
includes a support material having a surface pore size in the approximate range from
about 50 Angstroms to about 5000 Angstroms. The pore sizes should be sufficiently
large so that a permeate solvent can pass through the support without reducing the
flux of the composite. However, the pores should not be so large that the
permselective polymer membrane will either be unable to bridge or form across the
pores, or tend to fill up or penetrate too far into the pores, thus producing an
effectively thicker membrane than 200 nanometers. U.S. Pat. No. 4,814,082 (W. J.
Wrasidlo) and U.S. Pat. No. 4,783,346 (S. A. Sundet) are illustrative of methods of
choosing and preparing a porous base support for interfacial TFC (thin film
composite) membrane formation.
[0025] Non-limiting examples of the material forming the porous base support
include polysulfone, polyether sulfone, polyacrylonitrile, cellulose ester,
polypropylene, polyvinyl chloride, polyvinylidene fluoride and poly(arylether) ketones.
Other porous materials might be used as well, such as ceramics, glass and metals, in
a porous configuration. A wide variety of suitable porous base membranes are either
available commercially or may be prepared using techniques known to those of
ordinary skill in the art. In some embodiments, a porous base membrane which is a
polysulfone membrane or a porous polyethersulfone membrane are used because of
their desirable mechanical and chemical properties. Those of ordinary skill in the art
will be able to make the selection from among the suitable materials.
The thickness of the material forming the porous base support may be between
about 75 and about 250 microns thick, although other thicknesses may be used. For
example, a 25 microns thick porous base support permits production of higher flux
films. In some cases, the porous base support may be relatively thick, for example,
2.5 cm or more, where aqueous solution is applied to only one side, which is
subsequently contacted with the organic solution, forming the interface at which
polymerization occurs. The polymeric porous base support may be reinforced by
backing with a fabric or a non-woven web material. Non-limiting examples include
films, sheets, and nets such as a nonwoven polyester cloth. The polymer of the
porous base support may permeate through the pores, be attached on both sides of
the support, or be attached substantially on one side of the support.
[0026] In order to improve permeability and/or salt rejection, the thin film composite
membrane may be post-treated with an oxidizing solution, such as a sodium
hypochlorite solution. The concentration of sodium hypochlorite in the solution may
range from about 50 ppm to about 4000 ppm, and, in some embodiments, from about
50 ppm to about 500 ppm.
[0027] In processes according to the present invention, the organic solution or the
aqueous solution, or both the organic solution and the aqueous solution, may
include, in addition to the polyacid halide monomer or the polyamine monomer, multiwalled
carbon nanotubes dispersed therein. Suitable organic solvents include
aliphatic hydrocarbons, alcohols, ketones, esters, ethers, amides, and mixtures
thereof. In particular embodiments, aliphatic hydrocarbons such as decalins, and
isoparaffins, and mixtures thereof may be used. The organic solution may
additionally include a cyclic ketone such as cyclooctanone, cycloheptanone, 2
methylcyclohexanone, cyclohexanone, cyclohexene-3-one, cyclopentanone,
cyclobutanone, 3-ketotetrahydrofuran, 3-ketotetrahydrothiophene, or 3-ketoxetane,
particularly, cyclohexanone. Aqueous dispersions may include dispersing aids such
as polyvinylpyrrolidone, or surfactants, particularly non-ionic surfactants.
[0028] Carbon nanotubes have a cylindrical nanostructure with an inside diameter
(ID) and outside diameters (OD). While single-walled carbon nanotubes (SWNT) are
composed of a single layer of graphite in the shape of a tube or cylinder, multiwalled
nanotubes (MWNT) are made up of a single rolled layer of graphite or multiple layers
of graphite, arranged in concentric cylinders. The MWNTs for use in the processes
and membranes of the present invention have an outside diameter of less than about
30 n , particularly less than about 8 nm. The inner diameter of the multi-walled
carbon nanotubes is less than about 8 nm, and particularly useful are nanotubes with
inner diameters ranging from about 2 to about 5 nm. In the context of the present
invention, a WNT designated as having an outside diameter of less than about 30
nm, this means that greater than about 50% of the MWNT particles have an outer
diameter of less than about 30 nm, in some embodiments, more than about 75% of
the MWNT particles have an outer diameter of less than about 30 nm. Similarly,
when MWNTs are designated as having an outer diameter of less than about 8 nm, it
means that more than about 50% of the MWNT particles have an outer diameter of
less than about 8 nm, in some embodiments, more than about 75% of the MWNT
particles have an outer diameter of less than about 8 nm.
[0029] Concentration of the multi-walled carbon nanotubes in the organic solution or
the aqueous solution is at least 0.025% w/w, and may range from about 0.025% w/w
to about 10% w/w in some embodiments, and in others, ranges from about 0.025%
w/w to about 5% w/w. In yet other embodiments, the concentration of the multiwalled
carbon nanotubes ranges from about 0.05% w/w to about 1%w/w. In yet
other embodiments, the concentration of the multi-walled carbon nanotubes ranges
from about 0.1% w/w to about 1% w/w. In yet other embodiments, the concentration
of the multi-walled carbon nanotubes ranges from about 0.1% w/w to about 0.5%
w/w. The amount of carbon nanotubes contained in the final product ranges from
about 0.1 %-30% by weight in some embodiments; in other embodiments, from about
1%-1 0% by weight, and in still other embodiments, from about 0.5%-5% by weight.
[0030] In many embodiments, dispersions of carbon nanotubes in non-polar
hydrocarbons such as hexane, cydohexane, and isoparaffins are stable only for
short periods, even after prolonged sonication. Dispersion instability may be
minimized by incorporating an in-line, continuous mixer/ homogenizer. A dispersion
of nanotubes may be mixed with the monomer-containing solutions prior to use in the
coating operation using an in-line, continuous mixer/homogenizer in the processes of
the present invention in order to maximize stability of the nanotube dispersions.
Generally, a higher-volume stream of a solution of one of the monomers is mixed
with a lower volume stream of a carbon nanotube dispersion to form a new
dispersion containing both nanotubes and one of the monomers just before the
combined coating solution mixture is dispensed on the porous support membrane.
Suitable mixing/homogenizing devices include static mixers, ultrasonic mixers,
dynamic mixers, and other mechanical devices such as industrial mixers and
blenders with various types of blades, shafts, and impellers. Static mixers and
ultrasonic mixers are examples of preferred devices due to their simplicity and
effectiveness.
[0031] The nanotubes dispersion may be under constant or intermittent mixing
during the coating operation to ensure the homogeneous dispersion of nanotubes in
the coating solution(s). The mixing device includes (but not limited to) ultrasonic
mixing device, dynamic mixer, and other mechanical devices such as industrial
mixers and blenders with various types of blades, shafts, and impellers to make a
good quality homogeneous mixture. Ultrasonic mixing is one of the preferred
methods.
[0032] The advantage of separating the nanotube dispersion from the monomer
solution is that it decouples the various (and often conflicting) requirements of
nanotube dispersion stability and solvent compatibility with the porous support. For
example, conventional coating formulation (including conventional solvents such as
hexane and ISOPAR™G) may be used in the monomer solution(s), while more
aggressive solvents that are better at dispersing nanotubes may be used in making
the nanotube dispersion prior to combining the two. Since the residence time
between the in-line mixing and the coating is minimized, the nanotubes in the
dispersion do not have time to agglomerate and segregate. Also, since the solvent
used in dispersing nanotubes is typically a minor fraction in the final coating
formulation after the in-line mixing, the problem of solvent attacking the porous
support is resolved.
[0033] Coating methods typically include dip coating, slot die coating, and spray
coating. In some embodiments, when dip coating or slit die coating is used for both
aqueous and organic coating solutions, the coating tanks may be used as catch pans
to recycle the unused coating solutions. The nanotube dispersions may be in-line
homogenized outside the coating tanks and recirculated andreplenished during the
coating operation.
EXAMPLES
[0034] The following examples illustrate a process according to the present
invention.
General Procedures
[0035] Membrane Fabrication Using Handframe Coating Apparatus: Composite
membranes were prepared using a handframe coating apparatus consisting of a
matched pair of frames in which the porous base membrane could be fixed and
subsequently coated with the coating solution. The following procedure was used.
The porous base membrane was first soaked in deionized water for at least 30
minutes. The wet porous base membrane was fixed between two 8 inch by 11 inch
stainless steel frames and kept covered with water until further processed. Excess
water was removed from the porous base membrane and one surface of the porous
base membrane was treated with 200 grams of an aqueous solution comprising
meta-phenylenediamine (2.6% by weight), triethylamine salt of camphorsulfonic acid
(TEACSA) (6.6% by weight), the upper portion of the frame confining the aqueous
solution to the surface of the porous base membrane. After 30 seconds, the
aqueous solution was removed from the surface of the porous base membrane by
tilting the assembly comprising the frame and the treated porous base membrane
until only isolated drops of the aqueous solution were visible on the surface of the
treated porous base membrane. The treated surface was then exposed to a gentle
stream of air to remove isolated drops of the aqueous solution. The treated surface
of the porous base membrane was then contacted with 100 grams of an organic
solution containing trimesoyl chloride (0.16% by weight) and carbon nanotubes (type
and amount shown in examples) in ISOPAR™ G solvent. Prior to application of the
organic solution, the organic solution containing carbon nanotubes was first
sonicated using a bath sonicator (Branson 5510 model) for 60 minutes and then let
stand for 20 minutes. Excess organic solution was then removed by tilting a corner
of the frame and collecting the excess organic solution in a suitable collection vessel.
The frame was then returned to a horizontal position and the remaining film of
organic solution on the treated surface of the porous base membrane was allowed to
stand for about 1 minute. The remaining organic solution was drained from the
treated surface of the porous base membrane with the aid of a gentle air stream.
The treated assembly was then placed in a drying oven and maintained at a
temperature of 90°C for about 6 minutes after which the composite membrane was
ready for testing.
[0036] Membrane Performance Testing: Membrane tests were carried out on
composite membranes configured as a flat sheet in a cross-flow test cell apparatus
(Sterlitech Corp., Kent WA) (model CF042) with an effective membrane area of
35.68 cm2. The test cells were plumbed two in series in each of 6 parallel test lines.
Each line of cells was equipped with a valve to turn feed flow on/off and regulate
concentrate flow rate, which was set to 1 gallon per minute (gpm) in all tests. The
test apparatus was equipped with a temperature control system that included a
temperature measurement probe, a heat exchanger configured to remove excess
heat caused by pumping, and an air-cooled chiller configured to reduce the
temperature of the coolant circulated through the heat exchanger.
[0037] Composite membranes were first tested with a fluorescent red dye
(rhodamine WT from Cole-Parmer) to detect defects. A dye solution comprising 1%
rhodamine red dye was sprayed on the polyamide surface of the composite
membrane and allowed to stand for 1 minute, after which time the red dye was rinsed
off. Since rhodamine red dye does not stain polyamide, but stains polysulfone
strongly, a defect-free membrane should show no dye stain after thorough rinse. On
the other hand, dye stain patterns (e.g. red spots or other irregular dye staining
patterns) indicate defects in the composite membranes. The membranes were cut
into 2 inch x 6 inch rectangular coupons, and loaded into cross flow test cells. Three
coupons (3 replicates) from each type of membranes were tested under the same
conditions and the results obtained were averaged to obtain mean performance
values and standard deviations. The membrane coupons were first cleaned by
circulating water across the membrane in the test cells for 30 minutes to remove any
residual chemicals and dyes. Then, synthetic brackish water containing 500 pp
sodium chloride was circulated across membrane at 115 psi and 25 °C. The pH of
the water was controlled at pH 7.5. After one hour of operation, permeate samples
were collected for 10 minutes and analyzed.
[0038] After the initial test period, test coupons were exposed to a 70 ppm aqueous
solution of sodium hypochlorite at 25°C for 30 minutes. The test coupons were then
rinsed with deionized water for 1 hour.
[0039] Following the "chlorination" procedure, the test coupons were again tested for
reverse osmosis membrane performance with the synthetic feed solution containing
500 ppm sodium chloride used before as described herein. Solution conductivities
and temperatures were measured with a CON 11 conductivity meter (Oakton
Instruments). Conductivites were compensated to measurement at 25°C. The p
was measured with a Russell RL060P portable pH meter (Thermo Electron Corp).
Permeate was collected in a graduated cylinder. The permeate was weighed on a
Navigator balance and time intervals were recorded with a Fisher Scientific
stopwatch. Permeability, or "A value", of each membrane was determined at
standard temperatures (77°F or 25°C). Permeability is defined as the rate of flow
through the membrane per unit area per unit pressure. Avalues were calculated from
permeate weight, collection time, membrane area, and transmembrane pressure. A
values reported herein have units of 10- cm3/s-cm2-atm. Salt concentrations
determined from the conductivities of permeate and feed solutions were used to
calculate salt rejection values. Conductivitiesof the permeate and feed solutions
were measured, and salt concentrations calculated from the conductivity values, to
yield salt rejection values.
[0040] In some cases, the product composite membrane was rinsed with hot
deionized water and stored in a refrigerator before until testing or element fabrication.
In one case, the product composite membrane was treated with a solution containing
polyvinyl alcohol solution and then dried before storage, testing, or element
fabrication.
Comparative Example 1- :
[0041] A polyamide coated thin film composite RO membrane was fabricated using a
handframe coating apparatus An aqueous coating solution (Solution A) was prepared
and contained 2.6 wt% m-phenylene diamine (mPD) and 6.6 wt% triethylammonium
camphorsulfonate (TEACSA). An organic coating solution (Solution B) was prepared
and contained 0.16 wt% trimesoyl chloride (TMC) in ISOPAR™ G. A wet
polysulfone porous support film was first coated with the aqueous solution containing
the m-phenylenediamine (Solution A) and then coated with the organic solution
comprising the trimesoyl chloride (Solution B) to effect an interfacial polymerization
reaction between the diamine and the triacid chloride at one surface of the
polysulfone porous support film, thereby producing a thin film composite reverse
osmosis membrane. The product membrane was tested in triplicate using a solution
of magnesium sulfate (500 ppm in NaCl) at an applied operating pressure of 115
pounds per square inch (psi) and operating crossflow rate of .0 gram per minute
(grams per mole), at pH 7.0. The permeability and salt passage results are shown in
Table 1.
Comparative Example 1-2
[0042] Polyamide thin film composite RO membranes were fabricated as in
Comparative Example 1- 1 with the exception that the organic coating solution
(Solution A) also contained 0.1 wt% fullerene C60 (BU-602-BuckyUSA, Houston TX).
The product composite membranes were tested and membrane A-values and salt
passage properties were measured. Data are shown in Table 1. The data show that
coating solution contains fullerene C60 nano-particles showed no significant increase
in performance relative to a control (Comparative Example 1-1 ).
COMPARATIVE EXAMPLES 1-3
[0043] Polyamide thin film composite RO membranes were fabricated as in
Comparative Example 1-1 with the exception that the organic coating solution
(Solution A) also included 0.1% w/w single-walled carbon nanotubes (SWNT, P-3,
Carbon Solutions, Inc. Riverside, CA, I.D. - 1.4 nm, O.D. < 2 nm. The product
composite membranes were tested and membrane A-values and salt passage
properties were measured. Data are presented in Table 1- .
Comparative Example 1-4
[0044] Polyamide thin film composite RO membranes were fabricated as in
Comparative Example 1- 1 with the exception that the organic coating solution
(Solution A) further comprised 0.1 wt% multi-walled carbon nanotubes (1238YJS,
Nanostructured & Amorphous Materials, Inc., Houston, TX) with inner diameters of 5-
15 nm, outside diameters of 30-50 nm, and 0.5-2 m in length. The product
composite membranes were tested and membrane A-values and salt passage
properties were measured. Data is shown in Table 1.
Examples 1-1 and 1-2
[0045] Polyamide thin film composite RO membranes were fabricated as in
Comparative Example 1- 1 with the exception that the organic coating solution
(Solution A) further comprised 0.05% or 0.1 wt% w/w multi-walled carbon nanotubes
( 1225YJS, Nanostructured & Amorphous Materials, Inc., Houston, TX) I.D.2-5 nm,
O.D.< 8 nm, and 0.5-2 mh in length. The product composite membranes were tested
and membrane A-values and salt passage properties were measured. Data are
gathered in Table 1- . The data show that coating solution contains SWNT showed
had a significant increase in performance relative to the control without CNTs
(Comparative Example 1-1 ), controls with SWNTs (Comparative Example 1-3), and
controls with large OD (30-50 nm) MWNT (Comparative Example 1-4).
Dispersion Instability Comparative Examples and Dispersion Stability Examples
Comparative Example 2-1 : Instability of C T dispersions in ISOPAR™ G, hexane,
and cyclohexane
[0046] 0.01% single wall carbon nanotubes (P-3 from Carbon Solutions) were
dispersed in ISOPAR™ G by first sonicating for 60 minutes using a bath sonicator
(Branson 5510 model) in a glass vial with screw cap. Also, 0.01 wt% multiwall
carbon nanotubes with inside diameters of 2-5 n and outside diameters of less than
8 nm, and a length of 0.5 to 2 mm (1225YJS from Nanostructured and Amorphous
Materials, Inc) were dispersed in ISOPAR™ G, hexane, and cyclohexane. After the
sonication stopped, the dispersion instability was observed and the results are shown
in Figures 1 & 2 in previous section and Figures 3-5. These carbon nanotube
dispersions started to destabilize within minutes and were substantially segregated in
less than 20 minutes (Table 2).
Very poor: visible aggregation and phase segregation of CNT dispersion occurred
within 10 minutes after the sonication stopped.
Poor: visible aggregation and phase segregation of 0CNT dispersion occurred at
10-20 minutes after the sonication stopped
Fair: visible aggregation and phase segregation of CNT dispersion occurred at 20-
30 minutes after the sonication stopped
Good: visible aggregation and phase segregation of CNT dispersion occurred at 31-
45 minutes after the sonication stopped.
Excellent: No visible aggregation and phase segregation of CNT dispersion occurred
within 45 minutes after the sonication stopped.
Examples 2-1 through 2-2: Stability of CNTs dispersions in decalin mixtures.
[0047] 0.01 wt% multiwall carbon nanotubes (1225YJS) were dispersed in a decalins
or decalins/ ISOPAR™ G mixture by first sonicating for 60 minutes using a bath
sonicator (Branson 5510 model) inside a glass vial. After the sonication stopped, the
dispersion stability was observed and the results observed. These carbon nanotube
dispersions showed no visible segregation within 90 minutes. Thus the dispersion in
these decalin mixtures all showed excellent stability (Table 3).
Example 2-3: Stability of CNTs dispersions in decalin , cyclohexanone, and their
mixtures.
[0048] 0.1 wt% multiwall carbon nanotubes ( 1225YJS) were dispersed in a variety of
decalins/ ISOPAR™ G mixtures by first sonicating for 60 minutes using a bath
sonicator (Branson 551 0 model) in a glass vial. After the sonication stopped, the
dispersion stability was observed. The dispersions in these decalin mixtures showed
fair stability.
Examples 2-4 through 2-5 Stability of CNTs dispersions in cyclohexanone and
ISOPAR™ G
[0049] Multiwall carbon nanotubes ( 1225YJS) at 0.01 wt% loading were dispersed in
a variety of cyclohexanone/ ISOPAR™ G, mixtures by first sonicating for 60 minutes
using a bath sonicator (Branson 551 0 model) inside a glass vial with screw cap. After
the sonication stopped, the dispersion instability was observed. These carbon
nanotube dispersions showed no visible segregation within 30 minutes. Thus the
carbon nanotube dispersions in these decalin mixtures showed excellent stability
{Table 5).
Examples and Comparative Examples for Membrane Fabricated Using Handframe
Coating Apparatus
Comparative Example 2-6:
[0050] A polyamide coated thin film composite RO membrane was fabricated using a
handframe apparatus. An aqueous coating solution (Solution A, nominally 90.8 wt%
water) was prepared and contained 2.6 wt% meta-phenylene diamine (mPD), and
6.6 wt% triethylammonium camphorsulfonate (TEACSA). An organic coating solution
(Solution B) was prepared and contained 0.16 wt% trimesoyl chloride (TMC) in
ISOPAR™ G. Using a handframe apparatus and following the General
Polymerization Procedure described in the General Methods Section, a wet
polysulfone porous support film was first coated with the aqueous solution containing
the m-phenylenediamine (Solution A) and then coated with the organic solution
comprising the trimesoyl chloride (Solution B) to effect an interfacial polymerization
reaction between the diamine and the triacid chloride at one surface of the
polysulfone porous support film, thereby producing a thin film composite reverse
osmosis membrane. The product membrane was tested in triplicate as described in
this section using a solution of magnesium sulfate (2000 ppm in NaCl) at an applied
operating pressure of 225 pounds per square inch (psi) and operating crossflow rate
of 1.0 gallons per minute (gpm), at pH 7.0. The test results are shown in Table 6.
[0051] Following the test, the membrane was contacted with an aqueous solution
containing to 70 parts per million (ppm) of sodium hypochlorite at 25 °C for 30
minutes. The membrane was then rinsed with water for 1 hour, and then tested
again with the magnesium sulfate solution under the same conditions used
previously (2000 ppm NaCl, operating pressure 225 psi and operating crossflow rate
of 1.0 gpm, pH 7.0, ambient temperature to provide the data in Table 6 labeled
"Membrane A Value (after chlorination)" and "% Salt Passage (after chlorination)".
Comparative Examples 2-7 through 2-9
[0052] Polyamide thin film composite RO membranes were fabricated as in
Comparative Example 2-1 with the exception that the organic coating solvent
(Solution B) were made of decalins, 50:50 decalin/ ISOPAR™ G mixture, and 97:3
decalins/cyclohexanone mixture, respectively. The product composite membranes
were tested and membrane A-values and salt passage properties were measured.
Data are gathered in Table 7. The data show that when the organic coating solution
contains carbon nanotubes performance is enhanced relative to a control
(Comparative Example 2-6)
Comparative Examples 2-10 through 2- 2
[0053] Polyamide thin film composite O membranes were fabricated as in
Comparative Example 2-1 with the exception that the organic coating solution
(Solution B) further comprised 0.025, 0.05, and 0.1 wt% multiwall carbon nanotubes
(1225YJS). The product composite membranes were tested and membrane A-values
and salt passage properties were measured. Data are gathered in Table 8. The
data show that when the organic coating solution contains carbon nanotubes
performance was enhanced relative to a control (Comparative Example 2-6)
Table 8
Examples 2-6 through 2-7
[0054] Polyamide thin film composite RO membranes were fabricated as in
Comparative Example 2-1 with the exception that the solvent of the organic coating
solution (Solution B) comprising a 50:50 mixture of ISOPAR™ G and decalins
(Example 2-1 ) or 1 0% decalins (Example 2-3) and the solution also comprising 0.05
wt% (Example 2-1 ) and 0.1 wt% (Example 2-2) of MWCNT (1225YJS). The product
composite membranes were tested and membrane A-values and salt passage
properties were measured. Data are gathered in Table 9. The data show that when
the CNT was better dispersed in the organic coating solutions, performance of the
product composite membrane was enhanced relative to the controls (Comparative
Examples 2-6 and 9 ) . Comparative Examples 2-6 and 2-9 are included in Table 9
for convenience.
Table 9
Example 2-8
[0055] Polyamide thin film composite RO membranes were fabricated as in
Comparative Example 2-1 with the exception that the solvent of the organic coating
solution (Solution B) comprising a 97:3 mixture of ISOPAR™ G and cyclohexanone
and 0.05 wt% of MWCNT ( 1225YJS). The product composite membranes were
tested and membrane A-values and salt passage properties were measured. Data
are gathered in Table 10. The data show that when the CNT dispersion in organic
coating solutions are more stable, performance of the product composite membrane
is enhanced relative to the controls (Comparative Examples 2-3 and 2-4) containing
only one performance enhancing additive in either the aqueous or the organic
solution. Comparative Examples 2-6 and 2-1 2 are included in Table 10 for
convenience.
[0056] While only certain features of the invention have been illustrated and
described herein, many modifications and changes will occur to those skilled in the
art. It is, therefore, to be understood that the appended claims are intended to cover
all such modifications and changes as fall within the true spirit of the invention.

CLAIMS:
1. A process for manufacturing a thin film composite membrane comprising
multi-walled carbon nantubes, said process comprising contacting under
interfacial polymerization conditions an organic solution comprising a polyackJ
halide with an aqueous solution comprising a polyamine to form a thin film
composite membrane on a surface of a porous base membrane,
wherein at least one of the organic solution and the aqueous solution further
comprises multi-walled carbon nanotubes having an outside diameter of less than
about 30 nm, preferably less than less than about 8 nm.
2. A process according to claim 1, wherein the organic solution additionally
comprises multi-walled carbon nanotubes, and the aqueous solution is free of
multi-walled carbon nanotubes.
3. A process according to claim , wherein the concentration of the multi-walled
carbon nanotubes ranges from about 0.025% w/w to about 10% w/w,
preferably from about 0.025% w/w to about 5% w/w, and more preferably
from about 0.05% w/w to about 1% w/w.
4. A process according to claim 1, wherein the inside diameter of the multiwalled
carbon nanotubes is less than about 8 nm, preferably about 2-5 nm.
5. A process according to claim 1, wherein the polyacid halide is trimesoyl
chloride, and the polyamine is para-phenylene diamine.
6. A thin film composite membrane comprising a porous base membrane and a
polyamide coating disposed on said porous base membrane, said polyamide
coating comprising multi-walled carbon nanotubes having an outside diameter
of less than about 8 nm.
7. A thin film composite membrane according to claim 6, wherein the inside
diameter of the multi-walled carbon nanotubes is about 2-5 nm.
8. A thin film composite membrane according to claim 6, wherein the polyamide
coating is derived from trimesoyl chloride and para-phenylene diamine.
9. A desalination process comprising contacting seawater or brackish water with
a thin film composite membrane according to claim 6.
10. A process for manufacturing a thin film composite membrane comprising
carbon nanotubes, said process comprising contacting under interfacial
polymerization conditions an organic solution comprising a polyacid halide
and the carbon nanotubes with an aqueous solution comprising a polyamine
to form a thin film composite membrane on a surface of a porous base
membrane,
wherein the organic solution additionally comprises a saturated cyclic C -C o
hydrocarbon solvent, preferably at least one saturated polycyclic compound,
more preferably c/s-decalin, /rans-decalin, or a mixture thereof.A process
according to claim 1, wherein the organic solution additionally comprises at
least one saturated acyclic C4-C30 alkane compound, preferably an
isoparaffin.
11. A process according to claim 10, wherein the saturated cyclic C5-C2o
hydrocarbon solvent is c/s-decalin, frans-decalin, or a mixture thereof and the
saturated non-cyclic alkane is an isoparaffin.
12. A process according to claim 10, wherein the organic solution comprises
greater than about 20% w/w of the saturated cyclic C -C o hydrocarbon
solvent, preferably greater than about 50% w w of the saturated cyclic C 5-C20
hydrocarbon solvent, more preferably greater than about 80% w/w of the
saturated cyclic C -C20 hydrocarbon solvent.
13. A process according to claim 10, wherein the carbon nanotubes are multiwalled
carbon nanotubes.
14. A process according to claim 10, wherein the polyacid halide is trimesoyl
chloride, and the polyamine is para-phenylene diamine.
15. A process for manufacturing a thin film composite membrane comprising
carbon nanotubes, said process comprising contacting under interfacial
polymerization conditions an organic solution comprising a polyacid halide
and the carbon nanotubes with an aqueous solution comprising a polyamine
to form a thin film composite membrane on a surface of a porous base
membrane, wherein the organic solution additionally comprises a polysulfoneinsoluble
solvent having a density greater than about 0.8 kg/m3 and water
solubility of less than about 100 g/L, preferably c/s-decalin, frans-decalin.or a
mixture thereof.
16. A process according to claim 15, wherein the organic solution is a solvent
blend having a density greater than about 0.8 kg m3, and additionally
comprises a solvent having water solubility of less than about 100 g L.
17. A process according to claim 5, wherein the organic solution additionally
comprises a saturated non-cyclic C4-C30 alkane, preferably an isoparaffin.
18. A thin film composite membrane prepared by the process of claim 10.
19. A desalination process comprising contacting seawater or brackish water with
a thin film composite membrane according to claim
20. A composition comprising carbon nanotubes dispersed in an organic solution
comprising a solvent having a density greater than about 0.8 kg/m3and water
solubility of less than about 100 g/L.