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United States Patent [wj tm 4,199,448
JohESon et al. [45] Apr. 22, 1980
[54] REVERSE OSMOSIS MEMBRANE OF HIGH
UREA REJECTION PROPERTIES
[75] Inventors: Catherine C. Johnson, Los Altos
Hills; Theodore J. Wydeven,
Sunnyvale, both of Calif.
[73] Assignee: The United States of America as
represented by the Administrator of
the National Aeronautics and Space
Administration, Washington, D.C.
[21] Appl. No.: 694,407
[22] Filed: Jun. 9, 1976
[51] Int. Cl. 2 B01D 13/04; B01D 13/00
[52] U.S. a 210/23 H; 204/171;
210/500 M; 427/41; 427/245
[58] Field of Search 204/170, 171; 210/23 H,
210/321 R, 433 M, 500 M; 427/38, 39, 40, 41,
244, 245
[56] References Cited
U.S. PATENT DOCUMENTS
2,916,534 12/1959 Schallus et al 204/171 X
3,111,424 11/1963 Le Clair 427/38 X
3,600,218 8/1971 Pennebaker 427/38
3,662,046 5/1972 Woo et al 210/500 M X
3,847,652 11/1974 Fletcher et al 210/500 M X
3,925,187 12/1975 Bernard 427/38 X
Primary Examiner — Robert H. Spitzer
Attorney ; Agent, or Firm — Darrell G. Brekke; John R.
Manning; Armand McMillan
[57] ABSTRACT
Polymeric membranes suitable for use in reverse osmo-
sis water purification because of their high urea and salt
rejection properties are prepared by generating a
plasma of an unsaturated hydrocarbon monomer and
nitrogen gas from an electrical source and forming a
polymeric membrane by depositing a polymer of said
unsaturated monomer from said plasma onto a sub-
strate, such that nitrogen from the nitrogen gas is incor-
porated within the polymer in a chemically combined
form.
17 Claims, 1 Drawing Figure
1
4 , 199,448
REVERSE OSMOSIS MEMBRANE OF HIGH
UREA REJECTION PROPERTIES
ORIGIN OF THE INVENTION 5
The invention described herein was made by employ-
ees of the U.S. Government and may be manufactured
and used by or for the Government for Governmental
purposes without the payment of any royalties thereon
or therefor.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to polymeric mem-
branes which are characterized by high salt rejection 15
and high urea rejection properties. More particularly,
the present invention relates to membranes prepared by
the deposition of a polymeric material from an RF
plasma with the simultaneous incorporation of nitrogen
within the deposited membrane to form a polymeric 20
membrane of high urea rejection properties.
2. Description of the Prior Art
Advances in semipermeable membrane technology
have led to the development of many types of mem-
branes useful in reverse osmosis techniques for the puri- 25
fication and clarification of aqueous solutions. Many
membranes are known which perform effectively as salt
rejecting membranes in the purification of brine solu-
tions by reverse osmosis techniques. Moreover, in some
special applications such as the treatment of human 30
fluid wastes in prolonged manned space missions, it is
especially imperative that all waste water be purified to
a level suitable for human consumption. This means that
any potentially useful membrane for such applications
should not only be capable of salt rejection, but also 35
should be capable of rejecting urea if a portable water
filtered product is to be obtained.
Various techniques have been employed in the past
for the manufacture of semipermeable membranes
which include various casting methods such as spin 40
casting, dip casting, doctor blade casting, and the like as
well as other conventional techniques. The membranes
have been formed into various shapes such as sheets,
tubes, fibers and the like. In still another technique as
disclosed in U.S. Pat. No. 3,847,652 an RF plasma has 45
been used to form a polymeric film which is useful as a
salt rejecting reverse osmosis membrance. In the
method a liquid, ethyleneically unsaturated compound
such as allyl amine is subjected to an RF field on the
order of 30 to 50 watts in a vacuum of generally less 50
than 0.3 torr. The deposition of the polymeric mem-
brance can be accomplished in the presence of an addi-
tive gas such as nitrogen, helium, argon or the like,
which generally increases the rate of polymerization.
The resulting membrane, while possessing good salt 55
rejecting properties, is not very effective in rejecting
urea from aqueous waste solutions. In fact, its urea
rejecting abilities are comparable to those of the com-
mercially produced and widely used cellulose acetate
membranes which exhibit less than 20% urea rejection. 60
In another prior art technique as shown in U.S. Pat.
No. 3,775,308, a polymeric film is formed by subjecting
such liquid vinyl monomers as vinylpyridine, vinylpyr-
rolidine or the like to electrodeless glow discharge
polymerization. The polymeric membrane is deposited 65
on a porous substrate of a material such as cellulose
acetate, polysulfone, cellulose butyrate, or the like.
However, the membranes of the reference also lack
2
sufficient urea rejection properties. Furthermore, the
polymerizable vinylmonomers are liquid and conse-
quently it is difficult to control the addition of the mon-
omer to the reaction chamber.
A need, therefore, continues to exist for a polymeric
membrane which when used in the reverse osmosis
purification of aqueous solutions, exhibits high urea
rejecting properties.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to
provide a polymeric membrane which is characterized
by high urea and high salt rejection properties.
Another object of the present invention is to provide
a method for preparing a polymeric membrane of high
salt and high urea rejection properties.
Yet another object of the present invention is to pro-
vide an apparatus for the preparation of a polymeric
membrane.
Briefly, these objects and other objects of the present
invention as hereinafter will become more readily ap-
parent can be attained by a polymeric membrane char-
acterized by high urea and salt rejection properties and
prepared by generating a plasma of an unsaturated hy-
drocarbon monomer and nitrogen gas from an electrical
source and forming a polymeric membrane by deposit-
ing a polymer of said unsaturated monomer from said
plasma onto a substrate, such that nitrogen from the
nitrogen gas is incorporated within the polymer in a
chemically combined form.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention and
many of the attendant advantages thereof will be
readily obtained as the same becomes better understood
by reference to the following detailed description when
considered in connection with the accompanying draw-
ings, wherein:
The FIGURE is a diagram of the glass cross-reactor
employed in the present invention in which a polymeric
material is deposited on a supporting substrate.
DETAILED DESCRIPTION OF THE
PREFERRED EMBODIMENTS
The polymeric membrane of the present invention is
prepared by depositing a polymeric membrane on a
supporting substrate from a plasma formed by subject-
ing a gaseous mixture of nitrogen and an unsaturated
hydrocarbon monomer to an electrical discharge. The
polymeric membrane is characterized by the ability to
achieve greater than 80% urea rejection in the reverse
osmosis of aqueous waste solutions. In order to recover
potable water from urea containing aqueous solutions it
is necessary to employ a high urea rejection membrane
if several stages of filtration are to be avoided.
In the preparation of the present membranes nitrogen
gas and an unsaturated hydrocarbon monomer in the
gaseous state are mixed in a container and subjected to
an RF field. A polymer is deposited from the vaporous
RF plasma onto a porous substrate. Suitable unsaturated
monomers for the preparation of efficient urea rejecting
membranes include alkenes such as ethylene, propylene,
1 -butene, 2-butene, 1-pentene, 2-pentene, 3-pentene 1-
hexene, 2-hexene and the like; alkynes such as acety-
lene, propyne, 1-butyne and the like; aromatic, cyclic
and heterocyclic compounds such as styrene, pyrrole,
pyridine, and dienes such as 1,3 -butadiene, 1,3-pentadi-
4,199,448
5
nated by shutting off the source of power and by allow-
ing the reaction chamber to achieve atmospheric pres-
sure conditions. No precautionary measures are neces-
sary to prevent exposure of the deposited membrane to
ambient atmospheric conditions. 5
An important feature of the present invention relates
to the power which is applied by the RF generator to
form the plasma. The plasma in the present process is
subjected to a relatively low power compared to the
wattages employed in the processes disclosed by the 10
above-mentioned references. The maximum wattage
applied to the unsaturated monomer should not be
greater than fifteen watts. If the polymeric membrane is
deposited at a wattage greater than the upper limit, the
resulting membrane while possessing good salt rejection 1 5
properties, does not possess good urea rejecting charac-
teristics. The wattages employed in the processes of the
references, on the other hand, are substantially greater
than the wattages employed in the present process and
are typically about 48 watts for U.S. Pat. No. 3,847,652 20
and 30 watts for U.S. Pat. No. 3,775,308. Another fea-
ture which distinguishes the present process from those
of the prior art is the fact that the membranes of the
present invention can be prepared either within the
electric field of the plasma discharge or outside the 25
electric field. Confinement of the electric field to the
area between the electrodes determines whether or not
the membrane is deposited within or outside the electric
field. The membranes of the above patents however, are
only prepared outside of the electric field. 30
The types of apparatus and apparatus modifications
used in the process of the present invention is not criti-
cal, and those shown in U.S. Pat. No. 3,847,652 herein
incorporated by reference can be satisfactorily em-
ployed. Reference is hereby made to the accompanying 35
drawing which shows a preferred embodiment of the
reaction system of the present system. The reactor is a
glass cross 1 in which is positioned a pair of electrodes
3 and 5 in oppositely opposed arms of the cross. In the
embodiment shown the cross is a 10 cm diameter glass 40
cross. Electrode 3 is positive and is connected to exter-
nal radio frequency generator 7 by conductor 8 . Elec-
trode 5 is positioned opposite the positive electrode and
is connected to ground by conductor 9 . The electrodes
can be of any convenient size and in a preferred embodi- 45
ment are 9 cm O.D. copper electrodes. Each of the
openings on the four arms of the cross can be sealed
vacuum tight by placing teflon sealing gaskets 11 be-
tween the end of each arm 13 and a metal flange 15 .
Access into each arm of the cross can be achieved by 50
welding an appropriate metal rod onto the flange which
is usually formed of stainless steel or aluminum. Thus,
nitrogen from source 17 and unsaturated monomer from
source 18 are admitted through lines 19 and 21 respec-
tively by shut off valves 20 and controlled by mass 55
transducers 23 and flow control valves 25 . The gases
are mixed prior to entry into the glass cross in line 26
attached to the metal flange.
Pressure of the gaseous materials within the reactor is
controlled by the throttling valve 30 in line 31 placed 60
between the cross and vacuum pump 33 . The throttle
valve can be any conventional valve suitable for such
control purposes. The reactor is also provided with a
line 35 through which the system can be vented and
which is controlled by toggle valve 37. A cold trap can 65
optionally be placed within line 31 before the vacuum
pump. However, if a cold trap is used, the throttling
valve should be placed between the glass reactor and
6
the trap. By adjusting the throttling valve the flow rates
of the gases from the reaction chamber can be adjusted
thereby influencing the residence times of the gases in
the plasma. Slow flow rates on the order of 1-2 cc
STP/min. are maintained by the valve which results in
membranes that are tighter and more highly crosslinked.
These characteristics are necessary if the membrane is to
possess high urea rejection properties.
The electrodes positioned within the glass cross are
coupled to any convenient RF source. The electrodes
are capacitively coupled to the RF source rather than
inductively coupled.
The polymeric film or membrane of the present in-
vention can be used to purify aqueous waste solutions
containing not only salt but also urea. Thus, by reverse
osmosis procedures, conceivably the membrane can be
used for the purification of water in long term manned
space missions, for the recycling of water aboard sub-
marines or other vessels, for the purification of water
for Army field hospitals and the like. The pressures
employed in reverse osmosis procedures depends upon
the osmotic pressure of the liquid to be purified. Thus,
for instance, a pressure of 600 psi is commonly used for
the treatment of brine while a pressure of 1500 psi is
commonly used for treatment of sea water.
Having now generally described the invention, a
further understanding can be obtained by reference to
certain specific examples which are provided herein for
purpose of illustration only and are not intended to be
limiting unless otherwise specified.
The apparatus of FIG. 1 was used in the preparation
of certain membranes from a plasma of nitrogen and
ethylene. A series of membranes were deposited over a
period of 600 seconds at flow rates of 1-2 cc STP min.
for ethylene and nitrogen at a partial pressure of 0.2 torr
for each gas and a total pressure of 0.4 torr. The rejec-
tion and water permeation characteristics of each mem-
brane were measured and their characteristics com-
pared as a function of the power (watts) used in the
deposition of each membrane as shown in the table
below.
Power
Water Flux*
% Rejection
(Watts)
(Gall./Ft 2 -Day)
Urea
NaCI
15
0.14
85.2
93.2
25
3.0
47.1
95.7
35
13.3
24.4
94.4
45
10.9
16.6
84.1
55
57.7
7.8
45.0
'applied press
— 600 psi; feed solution =
10,000 ppm urea and 10,000 ppm NaCI
As is evident from the data in the table, the highest
urea rejection was achieved at 15 watts. As the power
was increased, the rejection properties of the mem-
branes decreased, A discharge could not be maintained
at less than 15 watts at a total pressure of 0.4 torr.
Having now fully described the invention, it will be
apparent to one of ordinary skill in the art that many
changes and modifications can be made thereto without
departing from the spirit or scope of the invention as set
forth herein.
What is claimed as new and intended to be secured by
Letters Patent is:
1. A method for preparing polymeric membranes of
high urea and salt rejection characteristics, which com-
prises:
4,199,448
7
generating a plasma of ethylene and nitrogen in an
RF field having a power within the range of about
15 to about 35 watts;
forming a polymeric membrane by depositing a poly-
mer of ethylene from said plasma onto a substrate, 5
such that nitrogen from the nitrogen gas is incorpo-
rated within the polymer in a chemically combined
form.
2. The method of claim 1, wherein said unsaturated
hydrocarbon monomer is an alkene, an alkyne, a cyclic 10
or heterocyclic compound.
3. The method of claim 1, wherein the flow rate of
nitrogen into said plasma ranges from 1 to 2 cc
(STP)/min.
4. The method of claim 1, wherein the flow rate of 15
said unsaturated monomer into said plasma ranges from
1 to 2 cc (STP)/min.
5. The method of claim 1, wherein the power of the
RF field which generates said plasma is 15 watts.
6. The method of claim 1, wherein the pressure of 20
said unsaturated monomer ranges from 0.1 to 0.4 torr
and the pressure of said nitrogen ranges from 0. 1 to 0.4
torr.
7. The method of claim 1, wherein said nitrogen and
unsaturated monomer are mixed prior to entry into said 25
plasma.
8. The membrane of claim 7, wherein said polymeric
material contains 15 to 22 wt % nitrogen.
9. The method of claim 1, wherein said plasma is
created by an internal capactively coupled discharge at 30
a radio frequency of 13.56 MHz.
10. A reverse osmosis membrane containing nitrogen
in chemically combined form prepared by the process
of claim 1.
11. A process for purifying aqueous wastes contain- 35
ing salt and urea, which comprises:
subjecting said aqueous waste to reverse osmosis over
a membrane prepared by the process of claim 1.
8
12. An apparatus for the plasma activated polymeri-
zation of unsaturated monomers in the presence of ni-
trogen, which comprises:
a glass cross reactor comprising four glass tubes,
wherein an open end of each tube is joined to form
a common center for all four tubes;
a pair of electrodes disposed within said reactor each
through one of the four tubes of the reactor and
sealed from the external environment of the reactor
and coupled to a radio frequency source so as to
generate a plasma within the reactor;
means for introducing and controlling the inflow of a
gaseous mixture of an unsaturated monomer and
nitrogen into the reactor through an unoccupied
sealed tube of the reactor;
means for evacuating the glass cross reactor; and
means for controlling the pressure of the gaseous
mixture within the reactor, wherein said evacuat-
ing and controlling means are attached to said reac-
tor through the remaining tube of the reactor.
13. The apparatus of claim 12, wherein one of said
electrodes functions as a support within the reactor for
collecting said polymerized material and is at ground
potential,
14. The apparatus of claim 12, wherein said plasma is
created by a radio frequency of 13.56 MHz.
15. The apparatus of claim 12, wherein said pressure
control means is a throttling valve.
16. The apparatus of claim 12, wherein each of said
electrodes is a 9 cm O.D. copper electrode.
17. The apparatus of claim 12, wherein said means for
introducing and controlling a gaseous mixture into said
reactor is a sequence of a shut off valve, a mass flow
tranducer and flow control valve each in a line from a
source of nitrogen and unsaturated monomer which
lines are joined together prior to entry into said reactor
to mix said nitrogen and unsaturated monomer.
* # * * *
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