Polymeric membranes suitable for use in reverse osmosis 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. A polymeric membrane is formed by depositing a polymer of the unsaturated monomer from the plasma onto a substrate, so that nitrogen from the nitrogen gas is incorporated within the polymer in a chemically combined form

Johnson, C. C.

Wydeven, T. J.

English

NASA Technical Reports Server

d States Patent 
Johnson et al. 
4,199,448 
1451 Apr. 22, 1880 
1541 
1731 
12 11 
REVERSE OSMOSIS MEMBRANE OF HIGH 
UREA REJECTION PROPERTIES 
Inventors: Catherine C. Johnson, Los Altos 
Hills; Theodore J. Wydeven, 
Sunnyvale, both of Calif. 
Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
Appl. No.: 694,407 
Filed: Jun. 9, 1976 
Int. c1.2 ....................... BOlD 13/04; BOlD 13/00 
US. c1. ................................. 210/23 H 204/171; 
210/500 M; 427/41; 427/245 
Field of Search ............. 204/170, 171; 210/23 H, 
210/321 R, 433 M, 500 M; 427/38, 39,40,41, 
244, 245 
ms ROW 
TRANSDUCER 
1561 References Cited 
U.S. PATENT DOCUMENTS 
2,916,534 12/1959 Schallus et ai. ................. 204/171 X 
3,111,424 11/1963 Le Clair ............................. 427/38 X 
3,662,046 5/1972 Woo et al. .................. 210/500 M X 
3,925,187 12/1975 Bernard ............................. 427/38 X 
3,600,218 SA971 Pennebaker ............................ 427/38 
3,847,652 11/1974 Fletcher et al. ............ 210/500 M X 
Primary Examiner-Robert H. Spitzer 
Attorney, Agent, or Firm-Darrell G. Brekke; John R. 
Manning; Armand McMillan 
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. 
[571 ABSTRACI 
17 Claims, 1 Drawing Figure 
VACUUM 
VENT 
https://ntrs.nasa.gov/search.jsp?R=19800014962 2020-03-21T03:10:42+00:00Z
4,199,448 
1 
REVERSE OSMOSIS MEMBRANE OF HIGH 
UREA REJECTION PROPERTIES 
ORIGIN O F  THE INVENTION 
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 O F  THE INVENTION 
1. Field of the Invention 
The present invention relates to polymeric mem- 
branes which are characterized by high salt rejection 
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 
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- 
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 
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 
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 
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 
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 R F  field on the 
order of 30 to 50 watts in a vacuum of generally less 
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 
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- 
merciallv Droduced and widelv 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 
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. 
5 
10 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 
l5 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- 
25 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 
3o nitrogen gas is incorporated within the polymer in a 
chemically combined form. 
2o 
BRIEF DESCRIPTION OF THE DRAWINGS 
A more complete appreciation of the invention and 
35 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 O F  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 
50 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 
55 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 
40 
45 
& R F  field. A polymer is deposited from the vaporous 
R F  plasma onto a porous substrate. Suitable unsaturated 
monomers for the preparation of efiicient 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, l-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 6 
nated by shutting off the source of power and by allow- the trap. By adjusting the throttling valve the flow rates 
ing the reaction chamber to achieve atmospheric pres- of the gases from the reaction chamber can be adjusted 
sure conditions. No precautionary measures are neces- thereby influencing the residence times of the gases in 
sary to prevent exposure of the deposited membrane to the plasma. Slow flow rates on the order of 1-2 cc 
ambient atmospheric conditions. 5 STP/min. are maintained by the valve which results in 
An important feature of the present invention relates membranes that are tighter and more highly crosslinked. 
to the power which is applied by the RF  generator to These characteristics are necessary if the membrane is to 
form the plasma. The plasma in the present process is possess high urea rejection properties. 
subjected to a relatively low Power compared to the The electrodes positioned within the glass cross are 
wattages employed in the Processes disclosed by the 10 coupled to any convenient RF  source. The electrodes 
above-mentioned references. The maximum wattage are capacitively coupled to the RF  source rather than 
applied to the unsaturated monomer should not be inductively coupled. 
greater than fifteen watts. If the polymeric membrane is The polymeric film or membrane of the present in- 
deposited at a wattage greater than the upper limit, the 
resulting membrane while possessing good salt rejection 
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 
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 
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. 
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 
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 
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- 
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 
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 
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 
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 
vention can be used to purify aqueous waste solutions 
j5 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- 
2o 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 
25 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 
30 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 
35 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- 
40 pared as a function of the power (watts) used in the 
deposition of each membrane as shown in the table 
below. 
45 Power Water Flux* % Rejection 
(watts) (Gall./Ft2-Day) Urea NaCl 
15 0.14 85.2 93.2 
25 3.0 47.1 95.7 
35 13.3 24.4 94.4 
10.9 16.6 84.1 
51.1 7.8 45.0 
50 :z 
'applied press = 600 psi; feed solution = I0,Mx) ppm urea andl0,Mx) ppm NaCl 
As is evident from the data in the table, the highest 
55 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 
60 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 
which is controlled by toggle-valve 37. A cold trap can 65 Letters Patent is: 
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 
I. A method for preparing polymeric membranes of 
high urea and salt rejection characteristics, which com- 
prises: 
4,199,448 
7 8 
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. 
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 electrodes is a 9 cm O.D. copper electrode. 
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. 
ing salt and urea, which comprises: 
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; 4. The method of claim 1, wherein the flow rate of 15 
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 
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 ffow control valve each in a line from a 
11. A process for purifying aqueous wastes contain- 35 source of nitrogen and unsaturated monomer which 
lines are joined together prior to entry into said reactor 
subjecting said aqueous waste to reverse osmosis over to mix said nitrogen and unsaturated monomer. 
a membrane prepared by the process of claim 1. ? s o , + *  
45 
50 
55 
60 
65 


Reverse osmosis membrane of high urea rejection properties

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