Note: Descriptions are shown in the official language in which they were submitted.
4 1 ~
2 --
This application is a division of co-pending application
Serial No. 343,311, entitled "Reverse Osmosis Membrane".
Tecllnical ~ield
This invcntion relates to pel~selective barri~rs
in the form of thin film composite membranes for
the selective separation of fluid mixtures and
solutions. An aspect of this invention relates
to polyamiaes (preferably ultrathin polyamides on
porous supports) suitable for reverse osm~sis
desalination of aqueous solutions containing dissolved
solutes, to the process for preparing these mem~rane
compositions, and to the process for using such
membranes.
Back~ro nd o~ the Prior Art
Reverse osmosis membranes have been prepared
~Lom a wide variety of ~no~n or preformed polymeric
materials. In preparation of such membr2nes the
polymer is typically dissolvea in a suitable solvent,
cast in the form of films or fibers, and auenched
in water to form asymmetric membranes. These
membranes include numerous polyamide and polyimide~
type membranes that show utility in reve~se osmosis
desalination processes~ In the book Reverse Osmosis
and Synthetic Membranes, National Research Council
of Canada, 1977, by S. Sourirajan, Chapter 9 by
P. Blais presen~s an extensive list of polyamide
~5 membranes, includin~ their fabrication and propertieS.
These polyamide membranes are additionally described
` in U.S. Patent Nos. 3,567,632, 3,600,350, 3,687,~2,
3,696,031, 3,878,109, 3,90~,519, 3,9~,823, 3,951,789,
and 3,993,625. These polyamide me~ran2s are
presently understood to be substantially linear
polymers synthesized in a prior operation, then cast
or extruded into permselective membranes -~y solvent
processes. Polyamide membr2nes made in this manner
appear to exhibit rather low fluxes to water in
reverse osmosis desalination processes, as lis-ted
in Table 2 of the abovc-cited hook, such that these
,
; ` ~
- 3 -
polyamide membrclnes have found practical use only
in the form of hollow fine fibcrs as accordincJ to
i the description in U.S. Patent ~o. 3,567,632.
In addi-tion, polyamide composite membranes,
suitable for use in reverse osmosis dcsalination
processes, have been prepared by condensation reactions
in situ on a preformed support film. Examplcs of
such membranes, and their preparation" are described
in U.S. Patent Nos. 3,74~,642, 3,951 ~15, 4,005,012,
and ~,039,440. For an example of an acid-catalyzed,
in sit~ polymerization on the support, see U.S.
Patent No. 3,926,798. For an example of the
crosslinXing of a preformed semi-permeable poly-
benzimidazole membrane, see U.S. Patent No. 4,020,142.
Permselective membranes made by this thin film
composite approach have, in some cases, exhibited
~reatly improved fluxes relative to preformed
polyamides subsequently cast or extruded into
membrane form by solvent processes. The aforemen-
tioned U.S. Patent No. 3,7~,642 contains a detailed
description of desalination membranes prepared by
interfacial condensation reactions.
~ lowever, the membranes of the prior art, whether
prepa~-ed from preformed polymers or by in situ
2S reaetions, have oftentimes exhibited one or more
othe~r deficiencies such as low salt rejection, low
d~l~abilit~ or resistance to compression, sensitivity
to extremes of pH or temperature, and lack of
resistance to microbial at-tack or oxidation by
ch~orine in the feed water. Lack of resistance
to chlorine in the feed water is a particularly
note~orthy de-Ficiency in permselective polyamide
membranes. According to U. S. Patent No. 3,951,815,
th~ site of attack by chlorine on polyamicle mclnbranes
is the amiclic hydroycn atom presel-t in the -~O--NT-I-
"
-- 4
group. In compositions such as the polypiperazine-amides
described in U.S. Patent Nos. 3,687,842, 3,696,031, and
3,951,815, resistance to chlorine in feed waters appears
to have been adequately demonstrated; however, such resistance
to attack by chlorine is believed to be atypical.
It would appear that permselective polyamide mem-
branes could be obtained by condensation polymerization of
diacyl halides with secondary diamines. Theoretically, the
resulting polymeric products would be devoid of amidic hydrogen
and would therefore be expected to be insensitive to chlorine
in the feed water. Representative polyamides such as this
have been prepared from piperazine and its derivatives as
described in U.S. Patent Nos. 3,687,842 and 3,696,031 and
NTIS Rept. No. PB253 193/7GA.
These compositions exhibited permeate fluxes of
2.4 to 650 liters per square meter per day (1/m2-day) at
a pressure of 80 atmospheres toward saline feed solutions
containing 5,000 to 10,000 ppm sodium chloride. These fluxes
are generally uneconomical for use in reverse osmosis desalin-
ation processes (except in the form of hollow fine fibers).
Currently, process economics indicate a need for
membrane fluxes of 600 to 800 1/m2-day at pressures of 55
to 70 atmospheres for seawater feed (35,000 to 42,000 ppm
total dissolved salts). For brackish waters containing 3,000
to 10,000 ppm salts, economically attractive membranes must
provide permeate fluxes of 600 to 800 l/m ~day at pressures
of only 25 to 40 atmospheres. While specific reverse osmosis
applications for permselective membranes may deviate from
these requirements, such membranes will no-t achieve broad
commercial applicability ~ _ n.. _.. ___.
4~
-- 5
unless they meet thcse criteria. A need ther~.~ore
remains ~or p2rmse~1ective polyamide mer~rane..t7ilich
combine the properties of chlorille resistar!ce a~d
high flux as ~ell as the other properties mentioned
above.
L~r~ t~
It has no~7 been discovered tha~ excellent
permselective membranes combining high flu~es with
chlorine resistance and lo~ salt passage can be
prepared b~ the interfacial condensa-~ion of poly-
unctional secondary amines ~7ith pol~functional
acid halides having an acid halide functionality
~reater than 2. It has fur~her been found that
mixtures of triacyl halides with diacyl halides
may provide syner~istic, unexplained flux-enhancing
effects on the permselective membranes in this
invention. It has also been discovered that superior
salt rejection properties can be achieved in the
permselective memkranes of this invention by forming
'amide prepolymers of the polyfunctional secondary
amines, subsequently employing these prepol~ers
in the interfacial preparation of the membrane barrier
`la~er.
In a preferred ernbodiment of this invention,
.5 the permselective or reverse osmosis membrane is
a composite comprising a microporous substrate and
~n "ultrathin" film having semipermeable properties
~eposited on one surface of said microporous substrate,
the ultrathin film being formed by contacting an
a~ueous solution of a chemical or chemicals contain-
.ing a m~ltiplicity of secondary amine functional
groups with vapors or a solution of an aromatic
trifunctional (or higher) acid halide alone or in
combination ~ith diacyl halidcs.
-
-- 6
Particularly good results are obtained with a thin film
composite permselective membrane which comprises a micro-
porous substrate (e.g. a conventional polysulfone support)
and a thin film having semi-permeable properties deposited
on one surface of the support, the thin film having been
formed by contacting an aqueous solution of aliphatic
heteroeyelie polyfunctional secondary amine (e.g. piperazine
or substituted piperazine) with a relatively nonpolar
solution of an aromatie triearboxylic acid halide alone or
mixed with an aromatic diearboxylie aeid halide, the solution
of polyfunctional acyl halides being capable of reacting
with the poly~unctional secondary amine to deposit the
resulting thin film on the support or substrate.
The invention is directed to a process for preparation
of a composite reverse osmosis membrane comprising the steps
of:
(a) coating a porous support with a layer com-
prising an aqueous solution containing, dissolved therein,
an essentially monomeric, polyfunctional, essentially water-
soluble secondary amine;
(b) contacting the layer with an essentially
water-insoluble, essentially monomeric, volatilizable poly-
functional acid halide component having an average acid
halide funetionality greater than 2.05, for a time suf-
fieient to effeet in-situ chain extension and crosslinking
reaetions between the seeondary amine and the polyfunctional
acid halide; and
(e) drying the produet of step (b) to form the
eomposite reverse osmosis membrane.
The porous support may comprise a polysulfone film.
The secondary amine may be selected from the group con-
sisting of piperazine and substituted piperazine. The acyl
halide may eo~rise trimesoyl chloride or a mixture of trime-
soyl chloride and isophthaloyl chloride. The polyfunctional
halide of step (b) may comprise isophthaloyl chloride.
The invention is also directed to a composite reverse
osmosis membrane prepared by the above process.
The invention is further directed to the improvement
comprising using the membrane as the reverse osmosis
membrane, in a process for desalination of saline water by
reverse osmosis comprising contacting the saline water under
pressure with a reverse osmosis membrane. The saline water
may eontain at least about 3,000 parts per million by weight
of an alkaline earth metal salt or a sulfate salt.
Definitions
Throughout this specification, the following terms
have the indicated meanings.
"Essentially monomeric" refers to a chemical compound
eapable of chain extension and/or cross-linking and/or other
polymerization reactions, which compound is relatively low
in molecular weight, is typically readily soluble in one
or more common liquid solvents, and is generally free of
repeating units linked by polyamide (-CO-NH-) linkages.
However, provided that the solubility in liquid alkane (including
halogenated alkane) solvents is not reduced below a fraction
of a pereent by weight, a very small number of repeating
polyamide units (e.g. two or three) can be present and the
compound ean still have some "essentially monomeric" character.
For example, in the case of a polyfunctional acid halide
monomer, the functionality can be increased by linking two
triacid halides with a difunctional chain extender (diamine,
diol, etc.) to form a tetraacid halide (or three triacid
halides to form a pentaacid halide, etc.).
~ "Essentially soluble" (e.g. "essentially water
soluble") denotes a measurable solubility in the solvent
whieh exceeds a desired level (e.g. greater than .01 wt.-%
or, more typically, greater than 1.0 wt.-%) under ordinary
eonditions of temperature and pressure (e.g. 20-25C. and
~S 1.0 atmosphere).
"Chain extension" refers to a type of chemical
reaction, preferably intermolecular in nature, which causes
the formation of a linear chain of repeating monomeric uni-ts
or increases the size of a polymeric, oligomeric, or prepoly-
meric molecular chain in an essentially linear fashion (i.e.
without necessarily increasing the crosslink density of the
` polymer, oligomer, or prepolymer).
- 7a -
"Nonpolar solvent" refers to solvents having a
polarity or dipole moment which is no greater
- ~ -
than the polarity or dipol~ moment of th~ low mole-
cular t~eight, liquid, halogenat~cl hydrocarbon solven-ts
(e.g~ d:ichloro.nethane). Accordingly, "nonpolar
solvents" are considerably less polar than the typical
polar`solvents such as wa-ter, Cl-C3 21}~anols, ammonia,
etc. and tend to be less than about 5 ~t.-~- soluble
in water at 20 C. Typi~cal "nonpolar solvents"
include the Cl-C12 aliphatic (incl~ing halo~enated
aliphatic) solvents such as the al};ane (includin~
halogenated al~ane) solvents having a relting point
bQlow 20 C. Non-aliphatic (e.g. aromatic) hydrocarbon
solvents are l~ss preferred. The most conveniently
used solvents are the Cl-C3 halogena~ed aliphatics,
the C5-C8 alkanes, C5 and C6 cycloaliphatics, etc.
"Secondary amine/acid halide ratio" refers to
the ratio of (a) equivalen-ts of secondary amine to (b)
equivalents of acid halide (i.e. the halide obtain~d
when one or more -OH functions of an acid such as a
carboxylic acid, cyanuric acid, or an inorganic
acid such as orthophosphoric acid are replaced with
halogen) in a reaction mixture. For exa~ple, in a
mixture o two moles of trimesoyl chloride (the acid
chloride of trimesic acid, 1,3,5-benzenetricarboxylic
acid) and three moles of piperazine, the secondary
?.5 amine~acid halide ratio would be 1:1 or stoichiometric,
while in an equimolar mixture of these t~io compo~1nds,
th~ sccondary amine/acid halide ratio ~o~ld be only
2:3 or 0.67:1 or l:l.S. A secondary amine/acid halide
~ . N~H/COCl) ratio of 1:1 (as in an e~uimolar
mixture of isophthaloyl chloride or terephthaloyl
chloxide and piperazine) tends to favor polyamide-
` forming chain extension, ~hile an N~/COC1 ratio
great~r than, say, 1.2:1 (e.g. 1.3:1 to 4:1), i.e.
an excess over stoichiometry of the ~ ~ component,
tends to favor the formation of secon~ary amine-
terminated polyamide prepolymers or oligomers having
.
_ 9 _
relatively fe~l repeatinq units lin~cd b~ polyaride
t-NlI-CO-) linkages and having a molecular ~eigrl~ in
the hundreds or low thousands (i.e. well belo;~
100,000). An NRH/COCl ratio in the neic~hborhood of
1:1 (e.g. 0.8:1 up to 1.5:1) tends to favor both
chain extension ancl crosslinkiny when the average
functionality of one of the components ~pref2rably
the -COCl component) is greater than 2.0 (e.g.
. > 2.05) and preferably about 2.1 to 3Ø The
:` 10 resulting molecular weight of the condensation
product (the polyamide~ is typically well above
100,000 and has a crosslink density in excess oE
one per 100,000. ~In prepolymer formation, it is
also preferred to make use of an averac3e acid haiide
functionality above 2.05, e.g. up to 3 or 4.~
The term "ultrathin" is defined in U.S. Patent-
No. 3,551,'44 (Forrester), issued December 29, 1970
as referring to films, membranes, or layers having
a thickness in the range of 0.05 to 5 microns (~ M).
For purposes o this invention, a 5-micron thickness
would be atypical, and thicknesses belo~ 1 micron
(1,000 m ~ M or 10,000 Angstroms) and even belo~ 0.5
micron (500 m~M) can provide good flux and good
salt rejection. Thic~nesses below 0.05 microns are
~` 25 dirficult to achieve in practice and hence not
ordinarily contemplated for ultrathin membrane
technology, but, theoretically, thicknesses do~;n
to perhaps 0.01 micron deposited on a suitable
porous support could impart salt rejection properties
to the membrane/support composite. Optimum flux and
salt rejection properties appear ~o be obta~ned in
the range of 0.05-0.3 micron (50-300 m~r~).
- Tne terms "polyamide prepolymer" and "polyamide
oligomer" are used more or less interchangeably to
denote relatively lo~J molecular ~eigllt polyamides
.
'
.
:
' ' . , : . ,. ::
:
.
Ls'3
-- 10 --
(e.g. as low as about 200 or 300 and typlcally not more than
a few thousand) having very few repeating units and/or very
few amide (-NH-CO-) linkages, e.g. as low as three amide
linkages per polyamide prepolymer molecule. The "polyamide
prepolymers" preferred for use in this invention are secondary
amine terminated, i.e., the number of terminal secondary
amine residues would be generally equal to the functionality
of the acid halide coreactant. For example, polyamide prepolymer
derived from piperazine and trimesoyl chloride would typically
have essentially three terminal piperazine residues. The
number of repeating units and the molecular weight of the
amine-terminated polyamide prepolymer is preferably kept
low enough to insure à reasonable level of water solubility.
"Volatilizable" indicates a solid or liquid compound
which can be boiled (preferably at normal atmospheric or
modestly reduced pressures such as 100 mm Hg) without substan-
tial chemical decomposition at temperatures below 1000 C.,
preferably below 500 C. Thus, isophthaloyl chloride, phos-
phorus oxychloride, trimesoyl chloride, etc. are "volatilizable".
~0 Detailed Description
The detailed description which follows is directed
; not only to the invention claimed in the present divisional
` application, but also to the invention claimed in co-pending
Canadian patent application Serial No. 343,311, of which
the present application is a division.
It has been found, in accordance with the present
invention, that composite membranes characterized by high
flux, high rejection, particularly of divalent salts, and
good resistance to attack by chlorine can be prepared by
interfacial polymerization reaction of a layer or film of
a solution of the secondary amine on a porous support with,
~`~
~ .
- ]Oa -
for example, a triacyl halide or its mixture with diacyl
halides, particularly as exemplified by trimesoyl chloride,
i.e. 1,3,5-benzenetricarboxylic acid chloride, or
`` '~ .
4 il~
a mi~:ture of trimesoy] chloride alld isop'l~th~loyl
chloride. In the conduct of this interEacial reac~ion,
the acyl halide ~roups reac-t ~ith secondary amine
~roups to produce amide lin~ayes devoid Oc amidic
hydroyen. Reaction is instantaneous at the interface
because of the eY~ceptionally high reaction ra~e o~
acyl chlorides for amines. The three-pronge~
functionality of the triacyl halides leads to the
generation of a highly crosslinked, three-aimensional
polymeric network in the membrane. The membrane
material is thus a polymer approaching infinite
molecular weight ~e.g. well above 100,000 molecular
weight) which further is generally insoluble in
virtually any solvent that does not first seriously
degrade its molecular structure. Hot~ever, not all
of the acyl halide functional groups become bound
into amide linkages. A substantial proportion of the
acyl halide func-'ional groups are hydrolyzed by
water present in the amine reagent, generatiny carboxylic
acid ~roups or carboxylate salts. These carboxyl
~roups have been aiscovered to eYert surprising
! e-fects on the performance of the interfacial membrane,
in that they enhance flux and profoundly effect the
membrane's rejection of aqueous dissolved solutes.
Because of the highly crosslinked nature of these
compositions, resulting in their ultrahigh molecular
wei~ht and lack of solubility, these compositions
are not well suited for processing into membranes
by any conventional polymer processiny techniques
such as film castin~ from solvents or melt forming.
It h~s been found that this invention can be
prepared in the form of thin film composite membranes
simply and efficiently by a series o steps comprising
(1) application of the amine solution to th~ porous
3~ support, and preferably saturation of the support
with t~e solution, preEerably follo~ed by pressing
~ .
- 12 -
of the support to remove ~cess aminc solution, ~2)
reaction tJith the acid halide, and (3) drying ~e.~.
air drying)
The porous support may be any of the types
conventionally used in reverse osmosis processes.
The prefe-r~d supports, however, are those prepared
from organic polymeric materials such as polysu'fone,
cnlorinated polyvinyl chloride, p~lyvinyl butyral,
polystyrene, cellulose esters, etc. Polysulfone
film has been found to be a particularly effective
suppor-t material for the membranes of the invention.
Preparation of such films is described in the above-
men-tioned U.S. Patent Nos. 3,926,79~ and 4,039,~40,
the disclasures of which are incorporated herein by
reference
The molecular formula o~ this invention can
be represented therefore by the following:
X~
where A = secondary diamine dicarboxamide,
~0 B ~ trifunctional organic moiety,
X = carboxylic acid or carboxylate salt, and
C = difunctional organic moiety derived
from a diacyl halide.
In the above structure, 1 + m must reside in the
25 r~nge of 2 to 100 percent o~ the polymer units,
and n may vary from O to 98~ of the polymer units.
In some cases, m may approach zero.
A typical embodime~t of the foregoing structural
formula can be prepared through interfacial reaction
of the monomeric amines with triacyl halides or
~` mixtures of diacyl and triacyl halides. Alternatively,
the same chemical composition can be prepared by
- '
:,
- 13 -
reaction of the amines with a tr:iacyl halide to form
an essentially water solu~le, amine-terminated
polyamide prepolyrner or oligomer, which then is
further polymerized by interfacial reaction with
diacyl or triacyl halides. As will subse~uently
be shown in the exemplilication of pre~errea embodimen-ts-
of the invention, the properties of the membranes o~
this invention can be widely varied depending upon
the choice of routes to the final polymer composition
and the ratios of acyl halides em~loyed.
The amine component in this invention may represent
any polyfunctional secondary amine of aliphatic or
cycloaliphatic origin to achieve chlorine resistance.
However, to achieve high membrane 1uY., selection of
piperazine (including piperazine derivatives) is
!' preferred in this invention as exemplified by
piperazine, 2-methylpiperazine, 2,5-dimethylpipera~ine.
The N,N'-dial~yl-substituted alkylene diamines and
linear polyethyleneimine are less preferred secondary
amines and are considered to be less effective than
the aforementioned piperazines. Use o~ mixtures of
secondary amines may be`prac-ticed under this invention,
including substantial amounts of mor.ofunctional
secondary arnines such as morphollne or dimethylarnine,
` 25 so long as the average ~unctionality o~ the secondary
amine mixture and the ratio of amine equivalents to
acid halide equivalents in the prepolyrner orming-
or polymer-forming reaction rnixture is in the desired
ran~e, whereby the final membrane composition consists
~` 30 of a high molecular weight, crosslinked, insoluble
polymer.
The higher-functionality acid halide ~e.g. the
triacid halide) should either be volatilizable
or essentially`soluble in nonpolar solvents such
as the aliphatic type ~including halogenated aliphatics
and cycloaliphatics). The acyl halides need not be
~ases or liquids at roorn te.-nperature (20-25 C.),
particularly if they can be melted or dissolved
readily or boiled wit-hout deconposition under
~. .
. . .
conveniently provided volatilization conditions r
since the vapor of compounds such as the polyfunc-
tional aromatic carboxylic acid chlorides, phosphorus
oxychloride, and cyanuric chloride (2,4,6-trichloro-
1,3,5-tria~ine~ is highly reac~ive ~ith secondary
amine solutions when such solutions are brought into
contact with the acid chloride vapor. CCyanuric
chloride can be considered to be the acid chloride
o~ cyanuric acid, C3N3[OH]3.) Finite, measurable
solubility in readily available solvents such as the
C5~C8 alkanes and the Cl-C3 halogenated,aliphatics
is particularly desirable, thoucJh the solutions
can be very dilute (down to 0.1% or even 0. 01o by
; weight) and total miscibility or even high'solubility
is not required. The acid halide should be selected
so that the acid halide functional groups ~e.g. -COCl)
will react'rapidly with secondary amines at
temperatures below 100 C., preferably below 30 C.
or even 25 C., the amide-forming reaction going
substantially to completion in a very short time
(e.g. less than a minute) under these conditions.
It is also pre~erred that the acid halide dissolved
in nonpolar solvent react readily at concentrations
`' of 0.01 to 10 wt.-%, particularly when the concentration
of the secondary amine in the aqueous medium of the
interfacial reaction system is in a similar range.
Thus, the acid halide should be selected ~Jith a
v:iew toward these solubility criteria and for
suitability in interfacial reaction systems, wherein
the secondary amine is in the aqueous phase and the
acid halide is in the nonpolar phase or a vapor phase.
Mixtures of triacyl halides may be used. Aromatic
~' acyl halides may be mononuclear or polynuclear
,` ~e.g. up to three aromatic rings), but mononuclear
' 35 compounds are prefe~red. Higher acyl halide
. .
`
- 15 -
functionality may be achieved by synthesizing partial adducts
of triacyl halides with difunctional reactants such as ethylene
glycol. Such adducts would fall within the scope of this
invention so long as they meet the aforementioned criteria.
The diacid halide which may be used in conjunction
with the triacyl halide may be taken from a list of compounds
including but not limited to oxyalyl chloride, succinoyl
chloride, glutaryl chloride, adipoyl chloride, fumaryl chloride,
itaconyl chloride, 1,2-cyclobutanedicarboxylic acid chloride,
orthophthaloyl chloride, meta-phthaloyl chloride, terephthaloyl
chloride, 2,6-pyridinedicarbonyl chloride, p,p'-biphenyl
dicarboxylic acid chloride, naphthalene-1,4-dicarboxylic
acid chloride, naphthalene-2,6-dicarboxylic acid chloride.
In virtually all cases, it is desirable that the diacid halide
meet the same criteria as the triacid halides described pre-
viously. Preferred diacyl halides in this invention are
mononuclear aromatic halides, particularly as exemplified
by meta-phthaloyl chloride and terephthaloyl chloride.
In one specific preferred embodiment of this invention,
; ~0 piperazine is employed in the form of an aqueous solution,
" with the concentration of piperazine being about 0.0 to 10%
~ by weight, preferably about 0.5 to 3%. Although the piperazine
i is, of course, itself an acid acceptor, the solution may
also contain additional acid acceptors, such as sodium hydroxide,
~5 trisodium phosphate, N,N'-dimethylpiperazine or the like
to facilitate the polymerization reaction. The presence ~```
of amounts of these is, however, generally not critical.
Application of the piperazine solution to the porous support
is readily accomplished by any conventional means such as
casting the solution on the support, dipping or immersing
.',` ' .
X
- 16 -
the support in the solution, or spraying the solution
on the sup~ort Generally, application of the piperazine
solution to the support is most conveniently and
efficiently accomplished by simply placing the support
in the solution for a time sufficient to permit
complete saturation of the support with the piperazine
solution. Removal of excess piperazine solution is
readily accomplished by conventional means such as
rolling or pressing at pressures sufficient to remove
excess solution ~Jithout damaging the support.
Reaction with the acid chloride, e.g. trimesoyl
chloride or a combination of trimesoyl chloride and
isophthaloyl chloride, is conducted under conventional
conditions ~or interfacial reaction between the
piperazine and the acid chloride. The acid chloride
is preferably employed in the nonpolar solvent, the
vapor phase contacting approach being ordinarily
less preferred. Concentration of the acid chloride
in the solvent will generally be in the range of
20 about 0.01 to 10% by weight, preferably about Q.l to
5%, with the wei~ht ratio of acid chloride-to
piperazine suitably being about 1:10 to 10~ enerally,
~` `room temperature is satisfactory for the reaction,
with temperatures of about 10 to 30 C. usuall~
giving good results.
Reaction with the acid;chloride is generally
~` most conveniently and efficiently accomplished by
simply i~mersing the porous support coated ~ith
` aqueous piperazine into the solvent solution of the
acid ehloride for a time sufficient to form a thin
coating of the resultiny poly(piperazineamide) on
the surface of the porous support. Generally, a
reaction time of about 1 second to 60 seconds is
.~ sufficient to form a thin coating or film of the
polymer possessing the desircd salt barrier
characteristics. The resulting composite, consisting
` - . ~ ' ` '
. .
- 17 -
of the porous support having a thin coating or film of the
poly(piperazineamide) on the surface thereof, is then air
dried, preferably at a temperature of about 20 to 130 C.
for a period of about 1 to 30 minutes, to form the composite
membrane of the invention. This membrane has been found
to exhibit high rejection for divalent salts, particularly
for magnesium sulfate, as well as high flux. Consequently,
the membranes are particularly useful for applications such
as brackish water desalting, whey concentration, electroplating
chemical recovery, softening of hard water for municipal
or home use, or for boiler feed water treatment.
` Where trimesoyl chloride alone is employed as the
acid chloride, the polymerization reaction is believed to
proceed primarily as follows:
1~ COCl _ _ _
+ ~ ~ N wate ~ ~ ~ C-N~_~N _ _ ~C
COCl COCl
f ~~' Im
~
. . . .
~a When isophthaloyl chloride is used as a co-reactant, the
polymerization reaction is believed to proceed essentially
; to the following:
f ~ c~L~ ~
Where a combination of the two acid chlorides is
employed, the mole ratio of isophthaloyl chloride to
trimesoyl chloride may ran~e up to about ~9 to
1, with a ratio of abou-t 2 to 1 generally giving
optimum results for maximurn ~ater fl~x, ana a ratio
of 9 to 1 generally ~iving an optimum combi~ation
of salt rejection and flux.
l~hen a co~bination of diacyl halide with a
triacyl halide is used, a large increase in the flux
of the resulting membranes is noted ~7hich would not
be expected based on the performance of membranes
made wi~h either the triacyl halide alone or the
diac~l h~lide alone. Permeate fluxes are observed
with these mixed acyl halide membranes which exceed
all published values for other membranes that exhibit
comparable salt rejection properties to~ards a salt
such as maynesium sulfate dissolved in the aqueous
feed. The m~mbranes of this invention also display
ion selectivity, in that rejection of multivalent
anions is uniforrnl~ very effecti~e, ~hile rejection
of monovàlent anions is dependent upon operating
conditions, the ionic strength of the feed water,
and the ratio of the diacyl and triacyl halides used
in the preparation of the membranes.
The invention ~ill be rnore specifically
illustrated by the following ~xamples, the first
of these Examples being a Control. Parts and
percentages are hy ~Jeight unless otherwise indicated.
Example l_ (Control)
This "Control" E~ample illustrates the performance
of a poly(piperazine isophthalamide) membrane,
described in U.S. Patent No. 3,696,031, but prepared
àccording to the art of thin film composite membranes.
This Exa~ple is used for comparison purposes.
A composite mernbrane ~as prepared by saturating
a polysulfone support film ~Jith a 1 wt.-~ aqueous
solution of piperar~ine (N ~ ~12), which
contained 1% NaO~I as an acid acceptor E~:cess
solution ~Jas removed by pressing the film with a soft
rubber roller. The saturated support ~^7as i~mersed
in a 1 wt.-% solution of isophthaloyl chloride in
hexane, at room temperature, for a period of 10
seconds. The drying and testing procedures for the
resulting composite membrane were the same as tho5e
of EYamples 2-6, and the flux and salt rejection
data are repor-ted hereinafter in Table 1.
Examples 2-6
Example 2 illustrates the greatly improved
m~mbrane flux performance achieved throu~h employment
of trimesoyl chloride and piperazine. ~xamples
3-6 demons.rate the unexpected, synergistic effect
of combinina the triacyl halide with a diacyl halide
in the preparation of the membranes of this invention.
;' ` A series of composite membranes were prepared
by the procedure described in Example 1 ~the Control).
In each case a polysulfone support film was saturated
with a 1 ~t.-% a~ueous solution of piperazine, which
also contained an additional acld acceptor as shown
in Table 1. Excess solution was removed by pressing
the film with a soft rubber roller.
The sa-turated support was then im~ersed in a
hexane solution of the acid chloride, the type and
concentrations of the acid chloride, in ~-~eight percent,
being ~iven in Table 1, at room temperature for a
! 30 pex-iod of 10 seconds. The support was then removed
from the reactant solution, drained and dried in air
at a temperature of 25~ C. for a period of 30 minu-tes.
The resultin~ composite membrane was tested in
a reverse osmosis cell for 24 ho~rs, using 0.5Oo
35- ~1gSO4 feed solution at 13.6 atmospheres and 25~ C.
.
- 20 -
P~esults are given in Table 1. It ~7ill be seen that
the me~brane p~epared from isophthaloyl chloride
alone tE~ample 1) sho~ed good salt rejection bu-t
very low flux, while the membrane pre~ared from
tri~nesoyl chloride alone ~Ex~mple 2) showed yood
salt rejQction and much improved flux. As is apparent
from the data of the Table, however, best results
were obtained from a combination oE isophthaloyl
chloride and trimesoyl chloride in suitable proportions
(E~amples 3, 4, and 5), resulting in excellen~
salt rejection as well as greatly improved flux.
Table 1
Magnesium
Ratio of Acyl Halides* Sulfate
Trimesoyl Isophthal~ lux Salt Rejection
E~ample Chloride Chloride _ l/m2d) (Percent)
1 0 1 155 9g.2
2 1 0 1060 9~.3 -
3 1.5 0,5 1260 99.9
~ 0.33 0.67 3140 99.6
0.2 0.8 2360 99.9
6 0.1 0.9 733 .99.0
*The presence of added acid acceptors varied depending
upon acid chlorides used: for example 1, 1% NaOH;
for Example 2, 1% N,N-dimethylpiperazine plus 0.2~
NaOH; for Examples 3 to 6, 2% Na3PO412H2O plus 0 5~ sodium
- dodecyl sul~ate.
Example 7
This Example illus-trates -the ion-selective
properties of composite membranes of this inven-tion.
A membrane was prepared according to Example 2.
~his membrane was mounted in a reverse osmosis test
cell and e~posed sequentially -to a series of a~ueous
salt solutions for a period of 20 to 24 hours per
aqueous test solution. Operating conditions ~ere
13.6 atmospheres pressure and 25 C. The flux and
salt rejection data for this membrane to-~ard the
various solutiolls, ~hich exhibits selectivity in
41~
- 21 -
the rejection o~ salts containiny divalent anions,
' arç shown in Table 2.
Table ~
Reverse Osmosls Test Data
5 Solutions Used in ~lu~ Salt Rejection
Reverse Osmosis Test (l/rn2d) (Perc~n-t)
. 0.1% ~IgSO~ 1430 98.0
0.5% NaCl 1710 50
0.5~ Na2SO4 1670 - 97.8
0.5~ MyC12 1300 46
0.5~ MySO~ 1300 97.9
,~ E~amples 8-11 .
These Examples illustra-te the utility of this
membrane invention in the treatment of various ~ater
` 15 sources.
.` Example 8: A membrane was prepared according
: to Example 6, and was mounted in a reverse osmosis
, test cell. Tests were run with a 3.5% synthetic
sea~later feed ,(made with a s~nthetic sea salt from
`I,ake Products Co., St. Louis, Missouri) and with a
synthetic brackish feed of the following composition:
CaC12-2~I2O 5.3 g/l
gSO4-7H2o 8.6 g/l
NaCl 10.4 g/l
25Na2S4 10.0 g/l
:.~ Na~CO3 0.2 g/l .
total dissolved solids 0.288~.
This membrane exhibited 9~.5% salt rejection
and 1030 1/m2d at 68 atmospheres when tested toward
synthetic seawater at 25 C. Toward synthetic
brac~ish water at 40.8 atmospheres and 25 C., this
membrane exhibited 94.6~ salt rejection and 1340
` l/m2d flux.
E~:ample 9: A membrane prepared according to
E~am~le 2.was tested for water so~teniny application
.
.
- 22 -
using a tap water charac-teristic of "hard" ~ia~-~r
containing magn~sium and ca~cium salts, said t,at2r
; having a conductivity of 0.53 x 10 mho. Th_
membrane produced 900 1/m2d and g~Q conducti~rit~r
rejection at 13.6 atmosphel^es and 25~ C., produc~ng
a "soft" water suitable for household use.
Example 10: A membrane was pr~pared acco-ding
to Example 4 and was tested for ~ater sof~ening
applications using a "hard" tap water as described
in Example 9. ~his membrane exhibited 2360 1/m2d
and 95.0% conductivity rejection at 13.6 atmos~heres
and 25 C., producing a "soft" water suitable for
household use.
~ ample 11: A membrane was prepared according to
Example 2. This membrane when tested under reverse
-~ osmosis conditions with 0.1~ magnesium sulfate at
13.6 a-tmospheres and 25 C. exhibited 1300 1/m2d
;- flux at 98% salt rejection. This membrane was
immersed in 100 ppm aqueous chlorine as sodium
hypochlorite for 72 hours and then was retested to
aet~rmine its stability toward chlorine attack.
It exhibited a flux of 1380 1/m2d at 97.4~ salt
rejection under the same conditions as before. This
E~ample illustrates the chlorine resistance of this
~5 type of membrane.
Examples 12-15
Use of Amine-Terminated Polyamide
-
Prepolymer ~ntermedia es
E~ample 12: An amine-terminated prepolymer of
3~ piperazine ~ith trimesoyl chloride ~Jas prepared 2S
follows. A solution of 2 grams (23 mllliequivalents)
of trimesoyl chloride in 100 milliliters of
1,2-aichloroethane was added over a 5 minute period
wi`th rapid stirring to a solution of 2 grams of
piperazine t~7 meq) and 2 grams triethylamine, present
- 23 -
as acid accc-~lor for n~utrali~ation, in 100 ml
dichloroethane. (The NR~I/COCl ratio was 2. o~
The prepolymer precipitated from the dichloroethane
~` during the reaetion. The prepolymer ~JaS filtered
5 off, washed with dichloroethane, and air dried to
yield appro~imately 4 grams ol product The prepolymer
~ was mixed ~ th 100 milliliters of water and stirre~
;~ for one hour at 60n C This solution was filterea
to remove insolu~le and gelatinous res~dues from the
prepolymer. Two grams of sodi~m hydroxide were added
to the clear filtrate. A microporous polysulfone
support was coated wi-th this solution, pressea with
a rubber roller ~o xemove excess solution, then
exposed to a 0.1% solution of isophthaloyl chloride
in hexane for 10 seconds, and finally air dried at
room temperature. This membrane when testea against
3.5% synthetic seawater at 6~ a~lospheres and ~5~ C.
~xhihited 430 1/m2d at 99.0~ salt rejection. Althou~h
higher flux (e.g. 600 1/m2d) would be greatly preferred,
~0 flu~ was ~rea.ly superior to some prior art membranes,
and a salt rejection capability above 9~rO is considered
a significant achievement in this art.
E~ample 13: A solution of 2 grams (23 meq) of
trimesoyl chloride in dichloroethane was added over
~5 lS minutes with rapid s-tirring to a solution of 1.5
grams (35 meq) piperazine and 0.5 grams ~6 meq)
morpholine in 150 milliliters of dichloroethane.
t-N~I/COCl ratio = 1.78:1, including -NRH ~ontributed
by morpholine.) The resulting suspension of prepolymer
in dichloroethane was stirred for 30 minu~es, followed
by filtrations, washing, and air drying of the
prepolymer to yield approximately 4 grams of product.
The prepolymer ~as dissolved in 100 milliliters of
water and filtered to remove a small amount of
gelatinous ma,erial. T-~o grams of l~ dimethylpiperazine
,
.
- 2~ -
acid acceptor ~as add~d to the solution. A microporous
polysulf~ne substrate was coated ~.~ith the solution,
pressed with a rubber roller to remove excess
solutionl exposed to a 0.1% solution of isophthaloyl
chloride in he~ane for lO seconds, then dried a~
130 C. in a circulating air oven for 15 minutes.
Tested against synthetic seawater at 6~ atmospheres
and 25 C., this membrane exhibited 1750 l/m d at
93.5% salt rejection.
Example 14: A dichloroethane solution of 2.0
~rams (33 meq) cyanuric chloride was added with
rapid stirring to a solution of 2.8 grams (65 meq)
piperazine. The prepolymer was filtered, ~ashed, and
dried. (-NP~/COCl ratio = 1.97~ hen mixed ~ith
lS water this prepolymer con-tained much water insoluble
material which was filtered off. The clear filirate
was neutralized with sodium hydroxide, followed by
addition of 0.5 gram more sodium hydroxide. The
solution was again filtered, providing 50 ml of clear
filtrate. A membrane was prepared by interfacial
rcaction of the ~mine prepolymer with isophthaloyl
chloride as described in Example 13, then dried at
130~ C. in an oven. The resulting membrane exhibited
570 l/m d at 99.2% salt rejection ~hen tested à~ainst
2~ synthetic seawater at 68 a~mospheres and 25 C.
Exa_~le 15: A solution of 2.0 grams (39 meq~
}~hospllorus o~ychloride in lO0 milliliters of
dichloroethane was added to a solution of 2.2 grams
(rl meq) o~ piperazine in lO0 milliliters of
dichloroethane with rapid stirring. (-NRH/acid halide
rc~tio = 1.31:1.) The prepolymer precipitate ~as
recovered and air dried. This prepolymer was conpletely
c.oluble ~7hen dissolved in lO0 milliliters of water
containing l.0 gram of sodium hydroxide. An
interfacial membrane was formed on microporous polys~llfone
,
- 25 -
as described in Example 13, then air drie~ hen
tested against synth~tic sea~ra~er at 6~ ~trnosp~e~es
and 25 C., this membrane e~hibited lS30 1/m2d at
93.9% salt rejection.
Exarnple 16
A membrane was prepared accoraing to Example 4
except that 2,6-pyridinedicarboxyIic acid chloride
was used in place of isophthaloyl chloride. When
-~ tested against 0.5% aqueous magnesium sulfate solution
at 13.6 atmospheres and 25 C., this membrane e~nibited
1210 l/m d at 95~ salt rejection.
Example 17
A wet polysulEone substrate was saturated ~Jith
an aqueous solution containing 2% piperazine and 2~
morpholine by weight. This coated support was pressed
with a rubber roller, exposed to a solution of 1~
trimesoyl chloride in hexane for 10 seconds, then
dried at 130 C. When tested against synthetic
seawater at 68 atmospheres, this membrane demonstrated
860 1/m2d at S6% salt rejection.
Example_18
According to the method of Exarnple 17, but using
an aqueous solution of 1% piperazine and 5% diethanolamine,
a membrane was prepared which exhibited 1550 1/m2d
at 70~ salt rejec-tion toward synthetic seawater at
6S atl-nospheres~
