TITLE: ANISOTROPIC MEMBRANES AND PRODUCTION THEREOF
European Patent Application EP0036315 A3

ABSTRACT:
Abstract of EP0036315
There is disclosed, in one aspect, an improved highly anisotropic
membrane comprising a skin and a porous asymmetric support. The skin
contains pores which have an average pore diameter from about 0.005 to
about 3.0 microns and this asymmetric support comprises a reticulated
structure which contains pores which have pore sizes ranging from about
10 to about 20,000 times as large as the average pore diameter of the
pores of the skin. The membrane has a bulk porosity greater than about
70%. In another aspect, there is disclosed a process for preparing
these membranes. This process comprises casting a polymer dope while
the dope is in a metastable liquid dispersion condition. The
concentration of polymer in the polymer dope should be high enough to
produce a coherent membrane yet low enough to form a substantially
all-reticulated structure within the asymmetric support. The membranes
are characterized by high flow rates, excellent retention properties
and improved resistance to fouling.

INVENTORS:
Wrasidlo, Wolfgang J.

APPLICATION NUMBER: EP19810301074
PUBLICATION DATE: 12/23/1981
FILING DATE: 03/13/1981

ASSIGNEE: BRUNSWICK CORPORATION
INTERNATIONAL CLASSES: B32B5/32; B01D39/16; B01D65/10; B01D67/00; B01D69/02; B01D69/06;
B01D69/10; B01D71/28; B01D71/34; B01D71/42; B01D71/56; B01D71/68;
B32B5/14; B32B27/30; B32B27/36; C08J9/28; (IPC1-7): B01D13/04
EUROPEAN CLASSES: B01D67/00K14B; B01D31/00; B01D65/10; B01D69/02

CLAIMS:
CLAIMS:
1. ~ An improved highly asymmetric polymeric membrane comprising a skin
and a porous support, said skin containing pores which have an average
pore diameter of from about 0.005 to about 3.0 microns and said support
comprising a reticulated structure which contains pores which have pore
sizes ranging from about 10 to about 20,000 times as large as the
average pore diameter of the pores of said skin, said membrane having a
bulk porosity greater than about 70so.
2. The membrane of claim 1, wherein said polymer is polyarylsulfone.
3. The membrane of claim 1, wherein said polymer is polyimide.
4. The membrane of claim 1, wherein the ratio of the maximum pore size
to the average pore size within said skin is less than about
5. 5. The membrane of claim 1, wherein said membrane has a
substantially larger dirt-holding ability in one flow direction than in
the other.
6. A process for preparing improved highly asymmetric integral
membranes comprising a skin and a porous support, said process
comprising casting a polymer dope while said dope is in a metastable
liquid dispersion condition, the concentration of polymer in said
polymer dope being high enough to produce a coherent membrane yet low
enough to form substantially all reticulated structure within said
porous support, to produce a membrane whose skin contains pores which
have an average pore diameter of about 0.005 to about 3.0 microns and
whose support contains pores which have pore sizes ranging from about
10 to about 20,000 times as large as the average pore diameter of the
pores of said skin, said membranes having a bulk porosity greater than
about 70%.
7. The method of claim 6, wherein said polymer dope when left
undisturbed segregates into at least two liquid layers.
8. The method of claim 7, wherein said segregation occurs within two
weeks.
9. The method of claim 6, wherein said polymer dope is cast into a
quenching liquid.
10. The method of claim 9, wherein said quench liquid is completely
miscible with the main solvent of said polymer dope.
11. The method of claim 6, wherein said polymer dope includes a
non-solvent for said polymer.
12. The method of claim 11, wherein said membrane is made by liquid
quenching and said non-solvent is only partially miscible with said
quenching liquid.
13. The method of claim 6, wherein said polymer dope is close to its
critical temperature at the time of quenching.
14. The method of claim 6, wherein said metastable liquid dispersion
condition is reached by maintaining the temperature of the dispersion
above the critical temperature and then lowering said temperature below
said critical temperature by casting said membranes.
15. An improved highly asymmetric polymeric membrane comprising a skin
and a porous support, said skin containing pores which have an average
pore diameter of from about 0.005 to about 3.0 microns and said support
comprising a reticulated structure which contains pores which have pore
sizes ranging from about 10 to about 20,900 times as large as the
average pore diameter of the pores of said skin, said membrane having a
bulk porosity greater than about 70%, said membrane being composed of
at least one member selected from the group consisting of
polyarylsulfone, polyimide, polycarbonate, polyacrylonitrile,
polyquinoxalines, and polyquinolines.
16. The membrane of claim 15, wherein said skin comprises less than
about 5% of the thickness of said membrane.
17. The membrane of claim 15, wherein the pore size distribution within
said skin is less than about 3 and wherein said skin comprises from
about 1 to about 5 of the thickness of said membrane.
18. A process for preparing improved highly asymmetric integral
membranes comprising a skin and a porous support, said process
comprising casting a polymer dope while said dope is in a metastable
liquid dispersion condition, the concentration of polymer in said
polymer- dope being high enough to produce a coherent membrane yet low
enough to form substantially all reticulated structure within said
porous support, to produce a membrane whose skin contains pores which
have an average pore diameter of about 0.005 to about 3.0 microns and
whose support contains pores which have pore sizes ranging from
about-lO to about 20,00p times as large as the average pore diameter of
the pores of said skin, said membranes having a bulk porosity greater
than about 70%, said polymer being selected from at least one member of
the group consisting of polyarylsulfone, polyimide, polycarbonate,
polyacrylonitrile, polyquinoxalines, and polyquinolines.
19. The method of claim 18, wherein said polymer is polysulfone and the
concentration of said polymer in said polymer-dope is from about 6 to
about 13 by weight of the casting dope.
20. The method of claim 18, wherein said polymer dope is cast into a
quenching liquid which is completely miscible with the main solvent of
said polymer dope.
21. The method of claim 18, wherein said metastable liquid dispersion
condition is reached by maintaining the temperature of the dispersion
above the critical temperature and then lowering said temperature below
said critical temperature by casting said membranes.
22. The membrane of claim 1, wherein said membrane may be used with
support side upstream, and when so used, the volume of fluid which may
be passed through said membrane by the time the rate of flow through
the said membrane is reduced to. 50% of the initial value is at least
about 2 times as large as the volume of fluid which may be passed
through the same membrane used with its skin side upstream.
23. The membrane of claim 1, wherein said membrane may be used with
support side upstream, and when so used, the volume of fluid which may
be passed through said membrane by the time the rate of flow through
the said membrane is reduced to 50% of the initial value is at least
about 5 times as large as the volume of fluid which may be passed
through the same membrane used with its skin side upstream.
24. A method of adapting a highly asymmetric microporous membrane
comprising a barrier skin and a highly asymmetric support to obtain a
representative bubble point from the skin of said membrane when said
membrane is used with said support upstream comprising eliminating the
pores from the entire outer perimeter of said asymmetric support.

DESCRIPTION:
Anisotropic membranes and production thereof 1. Field of the Invention
This invention relates generally to membranes which are useful as
ultrafilters and microporous membranes useful in separating materials.
More particularly, this invention relates to improved integral
anisotropic membranes having a skin and a porous support.
2. Description of the Prior Art Polymeric membranes are well known.
These membranes may generally be classified according to the sizes of
particles which they retain or according to their pore size as either
ultrafilter membranes, which have the finest pores, or microporous, or
microfilter membranes which have coarser pores. The dividing line
between ultrafilter membranes and microfilter membranes is at
approximately 0.05 micrometres in pore size or smallest retained
particle.
Membranes may also be classified according to the porosity difference
or similarity of their two faces. Thlls, membranes may be classified as
symmetrical1 when the two faces have similar porosity or as
asymmetrical when the two faces differ in porosity.
An important characteristic of a membrane is its permeability to water
which is customarily cpred in nits of cm/min-psi which represents the
niaqroscopic velocity in cm/min at which water flows through the
membrane when the driving pressure is one psi. This permeability may be
measured by the volume of pure water which passes through a unit area
of membrane per unit time.
The flow of water through the membrane is, within wide- limits,
directly proportional- to the apply pressure. In general, the
permeability to water decreases as the retentivity of the membrane to
solutes increases, because smaller pores offer more resistance to flow.
This relationship, however, is not a simple one since the retentivity
depends on the single smallest pore encountered by the liquid in
passing the membrane, whereas the resistance to flow depends on the
cumulatilfe effect of all the pores through which the sample must pass.
Hence, membranes of similar olnte retention having uniform pores
throughout their entire thickness have lower permeabilities than those
whose retentivity is due to a thin skin having proper pore sizes
combined with a body or substrate of much larger pores.
In other words, symmetrical membranes offer more resistance to fluid
flow and therefore have slower flow rates compared to asymmetrical
membranes of similar retentivity.
In addition to their retention characteristics, membranes may be
characterized by their ability to resist plugging or their dirt-holding
capacity.
Plugging refers to a reduction of the filtration rate during the
filtering operation as a function of the amount of liquid passing the
membrane. In order to extend the lifetime of a membrane in a given
filtration operation, it is customary to prefilter the fluid through a
membrane or filter having higher flow rates and smaller retentivities,
but still the ability to reduce severe fouling, or blocking of the
final membrane filter.
Structurally, membranes vary greatly and may generally be classified as
either reticulated or granular. In the former, there is a
three-dimensional open net5©cork of interconnecting fibrous strands and
open interstitial flow channels. In the granular type structure,
however, incompletely coalesced solid particles called granules leave
an interconnected network of pores between them. Reticulated membrane
structures generally have a higher porosity than granular membrane
structures. (Porosity of membranes is defined as (1- the relative
density). This difference, in turn, is the ratio of the weight of a
given volume of membrane to that of the bulk polymer forming the
membrane).
Polymeric membranes are generally made by preparing a solution of the
polymer in a suitable solvent, forming the solution into a thin sheet,
a hollow tube or a hollow fibre, and then precipitating the polymer
under controlled conditions. Precipitation may be carried out by
solvent evaporation or by contacting the polymer solution with a
nonsolvent.
U.S. Patent No. 3,615who24 discloses a method of forming porous
polymeric membranes which are described as being highly asymmetric. The
membranes produced according to that method are only slightly
asymmetric, however, and have permeability to water which is only
slightly higher than that of symmetrical membranes of the same
retentivity.
Membranes may also be classified as either composite, supported or
integral. Composite membranes comprise a very thin retentive layer
attached to a proformed porous support. In a supported membrane, the
actual membrane is attached to a strong sheet material of negligible
retentivity. Integral type membranes are formed in one and the same
operation having layers of the same composition. These layers may have
very different properties, depending, in general, on whether the
membrane is symmetrical or asymmetric.
The search has continued for improved, highly asymmetric, membranes
having improved retention properties and enhanced flow rates as well as
for processes for producing these membranes. This invention was made as
a result of that search.

OBJECTS AND SUMMARY OF THE INVENTION Accordingly, it is a general
object of the present invention to avoid or substantially alleviate the
above discussed problems of the prior art.
A more specific object of the present invention is to provide improved
anisotropic membranes which may be used as ultrafilters or microfilters
and which have improved flow rates and dirt-holding capacities.
Another object of the present invention is to provide a process for
preparing these improved anisotropic membranes.
Still other objects and advantages of the present invention will become
apparent from the following summary of the invention and description of
its preferred embodiments.
In one aspect, the present invention provides an improved anisotropic
membrane. This membrane comprises a skin and a highly porous asymmetric
support.
The skin contains pores which have an average pore diameter of from
about 0.005 to about 3.0 microns and the asymmetric support comprises a
reticulated structure which contains pores which have pore sizes
ranging from about 10 to about 20,000 times as large as the average
pore diameter of the pores of the skin.
The lower part of the 10 to 20,000 asymmetry factor range corresponds
to membranes which have larger surface pores whereas the higher part of
the 10 to 20,000 range corresponds to membranes which have finer
surface pores. The membrane has a porosity greater than about 70%.
In another aspect, the present invention provides a process for
preparing the improved anisotropic membranes described above.
This process comprises casting a polymer dope while that dope is in a
metastable liquid dispersion condition. The concentration of polymer in
the polymer dope should be high enough to produce a coherent membrane
yet low enough to form a substantially all-reticulated structure within
the asymmetric support. This process produces the membrane described
above.
BRIEF DESCRIPTION OF THE DRAWINGS Figure I comprises two diagrams which
show the effect of variations in solvent/non-solvent concentration and
in temperature in arriving at the metastable region.
Figure II shows the effect of changes in nonsolvent concentration on
the flow rate and retention of the membranes of the present invention.
Figure III is a scanning electron microscope photomicrograph of a
membrane of the present invention.
Figure IV is a scanning electron microscope photomicrograph of a
membrane of the prior art.
DESCRIPTION OF TIIE Pj#FEkL#ED ENBODIMENTS The present invention
comprises, in one aspect, an improved process for preparing anisotropic
membranes which have a relatively dense skin and a relatively porous
asy...et--c sii#r'o#t by casting a polymer dope while the dope is in a
metastable liquid dispersion and the concentration of the dope is high
enough to produce a coherent membrane yet low enough to form a
substantially all-reticulated structure within the asymmetric support.
The preparation of membrane casting dopes involves well-known
principles of polymer solubility, including the concept of critical
miscibility conditions, particularly the critical miscibility
temperature, T . Generally, at temperatures above T , c a polymer is
completely miscible with a given solvent, whereas below that
temperature, there is a region of phase separation. In the limit of
infinite polymer chain length, T' is the so-called "theta" - or e c
temperature or condition, where the interaction forces between polymer
molecules and solvent molecules equal the interactions of polymer
molecules for other molecules of the same polymer.
Solvents for polymers may be categorized into two types. "Good"
solvents are those in which the interactions (forces) between the
polymer molecules and solvent molecules are greater than the forces of
attraction between one polymer molecule and another polymer molecule.
Poor" or "ideal" solvents are those in which the interactions between
the polymer and solvent are equal to the forces of attraction between
one polymer molecule and another polymer molecule. Very poor solvents,
which dissolve little or no polymer, are called non-solvents. These
definitions apply not only to pure systems but also to mixtures of
solvents as well as mixtures of polymers.
The region on a phase diagram in which polymer is not completely
miscible with solvent may be delineated as a function of temperature
and solution composition by a curve called the "binodal". On one side
of this binodal curve, the system is thermodynamically unstable and on
the other side it is stable. The unstable system close to the binodal
is believed to undergo phase separation by a nucleation and growth
mechanism. Inside the binodal there exists another region called the
spinodal At or close to the spinodal the mechanism of phase separation
may change to the so-called "spinodal decomposition" in which periodic
and network structures may form without nucleation.
Figure I illustrates schematically the binodal and spinodal curves.
Figure Ia shows the effect of temperature and concentration for a
polymer and solvent.
Line 10 is the binodal and line 11 is the spinodal. The concentration
of polymer in the casting dope must be high enough to produce a
coherent membrane (see dividing line 12) yet low enough to obtain
substantially all reticulated structure in the asymmetric support (see
dividing line 13). The dividing lines 12 and 13 have been established
for purposes of illustration only and do not represent actual polymer
concentrations. Polymers of the present invention may be cast in the
shaded area 14 of Figure Ia. Toward the binodal side of region 14,
ultrafiltration membranes (pore sizes of from about 0.001 to about 0.05
micrometres) may be produced whereas toward the spinodal side of this
region, microporous membranes (pore sizes of from about 0.05 to about 3
micrometres) may be produced.
The term "metastable liquid dispersion" in this specification refers to
the region in Figure I between binodal 10 and s#inodal 11. Figure Ib is
a phase diagram for the three component system
polymer-solvent-non-solvent at constant temperature. The numbers in
Figure Ib represent the same thing as the corresponding numbers (absent
the "a") in Figure Ia represent.
In Figure I, toward the left side of line 10 beside shaded area 14 is
the region of stable solutions.
If polymer dopes are cast in this region, prior art membranes are
produced. These membranes lack the reticulated structure of the
asymmetric support of the membranes of the present invention. Toward
the right side of line 11, Figure I, beside shaded area 14 of Figure I,
there exists the region of two segregated phases where membranes cannot
be produced.
When a polymer/solvent system is made to cross the binodal by, for
example, lowering the temperature or by, for example, the addition of a
non-solvent, phase separation generally occurs spontaneously as
evidenced by the appearance of marked turbidity, but phase segregation
(i.e., the appearance of two distinct bulk layers) is delayed. The
system becomes a liqid-liquid dispersion, whose stability depends
markedly on how far the system is within the binodal. When the system
is within but close to the binodal condition, the system (i.e., polymer
dispersion) may be stable for weeks in the sense that no visible
segregation takes place. When the system is further inside the binodal,
segregation may occur within hours or even minutes.
Since segre- gated phases are the thermodynamically stable forms of the
system, these dispersions are metastable. As the process for converting
these metastable dispersions takes only seconds or minutes, it has been
found that such dispersions may be used for the formation of membranes.
Furthermore, it has been found that this metastable liquid dispersed
state (between binodal 10 and spinodal 11) which is an essential part
of this invention, may be approached from the position of a state
comprising stable'segregated phases by redispersing (i.e., agitating)
those segregated phases.
In general, as the system approaches the spinodal from the binodal, the
turbidity of the dispersion increases. This is believed due, in part,
to the formation of larger dispersed liquid particles.
During quenching of the metastable dispersed polymer solution with a
suitable non-solvent, very rapid solidification occurs. While this
process of solidification is not yet fully understood, it is believed
that for the membranes of this invention, the mechanism of
solidification involves spinodal decomposition of the very unstable
system formed by the interaction of the coagulating liquid with the
casting dope. Because of the rapidity of the process, the process may
be carried out even with relatively low viscosity polymer dopes. It is
also believed that spinodal decomposition is favored by bringing the
casting dope close to its spinodal.
The critical importance of casting membranes from solutions within the
binodal is in the way the properties of resulting membranes are
effected as illustrated in Figure II. Figure II shows the water
permeability and retention of the protein Immunoglobulin G of membranes
produced from a dope containing 10 per cent polysulfone in
dimethylformamide with various amounts of the non-solvent methyl-l
butanol. It may be seen that an enormous change in water permeability
and solute retentivity occurs between about 14.4 and 14.6 per cent of
added non-solvent.
The binodal corresponds to about 14.5 per cent and it is after the
binodal is crossed that the water permeability of the resulting
membrane is optimized. The individual points in Figure TI show how
reproducibly the properties of the membranes may be controlled.
In addition to the requirement of a metastable
polymer-solvent-non-solvent casting dope, the relation ship of these
comnonents to the quench liquid is also important. The polymer must, of
course, be insoluble in the quench liquid and the solvent should be
miscible with the quench liquid (and vice verso) In fact, it is also
preferred that the solvent and quench liquid be completely soluble in
each other under membrane formation conditions. Water is a generally
preferred quench liquid for economic and environmental reasons,
although a mixture of 80 per cent water and 20 per cent dimethyl
formamide is particularly preferred. It is also preferred that the
solvent have a low viscosity.
If the solvent has a low viscosity and the solvent and quench liquid
are completely soluble in one another, rapid diffusion of the quench
liquid into the casting dope is assured.
The non-solvent, if present, should have only limited solubility in the
quench liquid. This limited solubility is believed to be effective in
increasing the asymmetry of the resulting membrane.
The asymmetry of a porous membrane may be measured by the ratio of the
average pore diameter of its two faces.
The formation of membranes having larger pores (above about O.2#im) may
be accomplished in various ways as discussed herein but, for
convenience, the process by which these large pore membranes are
produced is preferably facilitated by increasing the proportion of
non-solvent in the casting dope. At constant turbidity, the amount of
non-solvent that may be added to the system without causing spinodal
decomposition is higher at higher temperatures. For best results it is
preferred to increase the temperature very close to the critical
temperature for a given solvent/non-solvent combination as illustrated
in Figure IA and also as in Example VIII.
At lower temperatures, the proportions of solvent described are not
attainable.
The quench liquid as indicated above should be inert with respect to
the polymer used and should preferably be miscible with the polymer
solvent and should have limited miscibility with the non-solvent.
When water is used as the quench liquid, its properties and those of
the resulting membranes may be modified by the presence of certain
additives, such as surfactants and solvents. The addition of one or
more surfactants to the quench liquid often makes the membrane
hydrophilic so that it is easily wetted by water and thus may be used
for filtration without substantial pressure to overcome capillary
forces.
The amount of surfactant may vary widely but generally from about 0.001
to about 2 per cent, typically from about 0.02 to about 0.2 per cent,
and preferably from about 0.02 to about 0.1 per cent by weight total
quench liquid may be employed.
Typical surfactants include sodium dodecyl sulfate, ethoxylated
alcohols, glycerol esters and fluorocarbon surfactants.
The concentration of the polymer in the casting dope must be low enough
to form a substantially all reticulated structure uithin the asymmetric
support but must be high enough to produce a coherent membrane. If the
polymer concentration were too low, the resulting membrane would have
no coherency and, in the extreme case, only dust would be formed. If
the polymer con centration were too high, then the structure within the
asymmetric support would not he all substantially reticulated, but
would contain at least some granulated structure.
Although the appropriate concentration of polymer varies somewhat
depending upon the particular polymer used, the polymer concentration
when the polymer is polysulfone alone should be generally from about 6
to 13, typically from about 8 to about 12, and preferably from about 9
to about 10 percent by weight of the casting dope. The particular
polymer chosen will, of course, determine to a large extent, the final
properties of the membrane.
It will be evident from the above that this invention may be practiced
with a variety of polymers or their mixtures, solvents or their
mixtures, and nonsolvents or their mixtures and over a range of
temperatures, provided that the combination of these components and
parameters is such as to produce the desired metastability. The
properties and performance of the membrane produced by casting a
metastable dope depend not only on the degree of this instability and
on the general relationship of mutual solubility of the polymer,
solvent, non-solvent and quench liquid as outlined above, but also on
the particular materials selected and their mutual interactions in a
manner that is not fully understood at present.
The following are some of the materials which have been found useful in
the practice of this invention but it will be clear to those skilled in
the art that many others and/or their combinations may also be used.
The polymers which may be used to produce these membranes include
broadly any polymer generally used to produce membranes, although, as
will be discussed in greater detail hereinbelow, the choice of polymer
is related to the choice of solvent and nonsolvent used in the casting,
process. Polymers which have been found to be particularly useful in
the instant process include polysulfone polyimides, poly carbonates,
polyacrylonitriles including poly alkylacrylonitriles, polyquinoxalines
and polyquinolines.
Mixtures of two or more polymers may also be used.
Preferred polymers within the above noted groups for use in the present
invention include Ciba Geigy X U218 polyimide (which is a
solublethermoplastic polyimide based on
5(6)-amino-l-(4'-aminophenyl)-l,3, 3,- trimethylindone)), Up john P
2080 polyimide, Lexan polycarbonate, polyphenylquinoxaline, and Union
Carbide P-3500 polyarylsulfone.
A particularly preferred polymer for use in the presently claimed
invention is Union Carbide P-3500 polyarylsulfone. When this particular
polymer is employed, it has been found that a minimum molecular weight
of 30,000 is needed in order to obtain a coherent membrane with a
reticulated structure. The upper limit of molecular weight is
approximately one million. The use of polyarylsulfone with molecular
weights in excess of one million is undesirable because of the
formation of polymer gels. The molecular weight range of the other
polymers that may be useful in the presently claimed invention differs,
of course, depending upon the particular polymer employed.
The solvents which may be used to produce membranes according to this
invention include: Dimethyl formamide Dimethyl acetamide Dioxane
N-Methyl pyrrolidone Dimethyl sulfoxide Chloroform Tetramethylurea
Tetrachloroethane Suitable non solvents include: Methanol Acetone
Ethanol Methyl ethylketone Isopropanol Methyl isobutyl ketone Amyl
alcohol Nitropropane Hexanol Butyl ether Heptanol Ethyl acetate Octanol
Amyl acetate Propane Hexane Heptane Octane A significant advantage of
the membranes of the present invention is their high fluid
permeability, particularly for small pore sizes. This is believed to be
the result of their very high asymmetry so that the reticulated part of
the membrane offers a relatively low resistance to fluid flow as
compared to the finely porous skin.
For example, a membrane prepared according to this invention as
described hereinafter in Example VI has a pore size of 0.01 sm and a
flow rate of 0.9 cm/min psi, one hundred times higher than that of
commercial membranes now available having this pore size. At ~higher
pore sizes the advantage of membranes prepared according to this
invention is less striking but remains significant even at the large
0.4/cm pore size (Example VIII).
These membranes can provide a flow rate of over 8 cm/min psi which is
30 to 600·o greater than that of membranes now commercially available.
- A'log-log plot of the water permeability of the membranes of the
present invention against pore size indicates a less than inverse
relationship whereas the same plot of conventional membranes indicates
a much faster decrease.
The membranes of this invention may be used with either the skin side
or the support side of the membrane upstream with respect to fluid
flow.
For microporous membranes, however, it is preferred to use the membrane
so that the support is upstream, In this way, the related porous
support serves as a built-in prefilter, greatly increasing the
dirt-holding capacity of the membrane. The fluid encounters the largest
pores first and later encounters pores having gradually decreasing size
with the smallest pores - those in the skin - being encountered last.
Silence larger particles are retained before they reach the skin and do
not clog its pores.
An in-depth filtration is obtained in which the particles are retained
at various levels leaving many more pores available for flow then if
they were all retained in one plane at the skin. If the membrane is not
highly asymmetrical this advantage does not exist since approximately
the same amount of retained matter fouls both sides of the membrane
because the pore sizes on both sides are approximately the same.
When a microporous membrane of the present invention is used with its
support side ups team, it has been found that the volume of fluid which
may be passed through the membrane by the time the rate of flow through
the membrane is reduced to 50o¢o of the initial value is generally at
least about 2, typically from about 2 to about 6, and preferably at
least about 5 times as large as the volume of fluid which may be passed
through the same membrane used with its skin side upstream. For certain
applications, it is important that one be able to readily test the
integrity of a membrane. The so-called "bubble point" test may be used
to determine the largest pore size in the membrane.
This bubble point test involves passing a gas such as air, through a
wetted membrane and determining the pore size as a function of the
pressure needed to push a bubble through that pore of the membrane. It
is important that one obtain accurate bubble point data since there is
a correlation between bubble point and ability of a membrane to
sterilize a particular fluid. If the highly asymmetric microporous
membranes of this invention are used with the support side upstream,
the representative bubble point of the skin cannot be obtained because
the gas passes parallel to the skin out the sides of the asymmetric
support. Thus one could not use this bubble point test for highly
asymmetric membranes which are used with their support side upstream.
To overcome this problem, it has been discovered that one can eliminate
the pores from the outer perimeter of the asymmetric support by either
removing those pore or filling the pores by means of, for example,
sealing the outer diameter of the membrane, such as by heat sealing,
filling the pores with an impermeable substance, such as glue, or
mechanically collapsing the pores. By filling these pores or removing
these pores, the representative bubble point of the skin Xs obtained.
The present invention is further illustrated by the following examples.
All parts and percentages in the examples as well as in the
specification and claims are by weight unless otherwise specified.
Example I A 5% by weight solution of polymethacrylo nitrile in
dimethylformamide is prepared at 400C with rapid stirring. The solution
is degassed by placing it in a water bath at 400C for 30 minutes and
then cooled to 300C (the critical temperature). At this temperature the
solution becomes abruptly turbid.
This solution is spread by means of a casting knife to a wet thickness
of 10 mils onto a glass plate which is preheated to 30 C and the plate
is transferred immediately into a water bath at ambient temperature,
causing the wet film to coagulate. The resulting membrane has a water
flow rate of 0.5 cm/min psi and a pore size of .05 pm.
Example II A turbid solution consisting of 4% polyimide (P 2080 Upjohn)
and 1.2% polycarbonate (texan, very high molecular weight) in
dimethylformamide is made at 25 C, degassed for 30 minutes at 25 C, and
cast onto a glass plate at 25 C. The wet film is coagulated as in
Example I. This membrane has a flow rate of 1.2 cm/min psi and a
molecular weight 106 cut-off of 10 corresponding to a pore size of 0.1
pm.
Example III A 5% solution of polyphenylquinoxaline dissolved in a
mixture of 800/o chloroform and 20% mcresol at 15 0C results in,a
turbid solution which is then cast onto a glass plate cooled to 15 C.
The plate is rapidly transferred into a bath of isopropyl alcohol
cooled to 10 C. The wet film solidifies instantly to produce a membrane
having a flow rate of 3 cm/min psi and a pore size of 0.2 ssm.
Example IV A solution of polyquinoline (M.w 80,000) N in a mixture of
85% chloroform/158z·O methylene chloride turns cloudy after adding 16.2
i-butyl alcohol at 25 0C resulting in a final polymer concentration of
5 by weight. This solution is cast onto a flat polished stainless steel
plate at 250 C and placed into an isopropyl alcohol bath at ambient
temperature. The rapidly formed membrane has a flow rate of 30 cm/min
psi and a surface pore size of 1.2 micron.
Example V A solution of polysulfone (P-3500 Union Carbide) is prepared
in dimethylformamide and titrated with isopropyl alcohol to a
composition consisting of 9.5% polysulfone, 10.3% isopropyl alcohol and
80.2% dimethylformamide. This solution is cast at ambient temperature
(which is above the critical temperature) into a membrane as described
in Example I. This membrane has a flow rate of 4 cm/min psi and a pore
size of 0.2 pm, an elongation 2 at break of 24% and a tensile strength
of 30 kgfcm Example VI A mixture of polyimide (4%) and polysulfone (1%)
is dissolved in a 50/50 weight mixture of chloroform and cresylic acid.
The slightly turbid solution is cast onto a glass plate and quenched
into an isopropyl alcohol bath.
The resulting membrane has a flow rate of 0.9 cm/min psi and gives a
99.5 ,0 retention of ovalbumine.
Example VII A solution of# polysulfone (P-3500 Union Carbide) is
prepared in dimethylformamide and titrated with hexane to a composition
consisting of 9.9% polysulfone, 11.9% hexane, and 78.2%
dimethylformamide. This solution is cast into water at ambient
temperature resulting in a membrane with a flow rate of 0.32 cm/min-psi
and 985 retention of ovalbumine.
Example VIII A solution of polysulfone-{P-3500.Unjon Carbide) is
prepared in dimethylformamide and heated to 40 Cs This solution is then
titrated with 2-methyl -2-butanol to an absorbance reading of 0.600
corresponding to a concentration of 9.5% polysulfone, 15.5 ,6
2-methyl-2-butanol and 75% dimethylformamide. (The absorbance is zeroed
with a 12% polysulfone/88% dimethylformamide solution.) This solution
is cast at 4o 0C into water. The resulting filter has a flow rate 7.4
cumin psi, a bubble point of 35 psi and an average pore diameter of O,
ym.
Comparative Example I Figure III is a scanning electron microscope
photomicrograph of the top surface (magnified 100,000 times) and the
bottom surface (magnified 80 times) of a membrane (having a molecular
weight cut-off of 25,000) produced in accordance with the present
invention by casting a solution of 10% polysulfone in a 78/12 mixture
of dimethylformamide/hexane. The pore size of the asymmetric support is
approximately 20,000 times the size of the pores in the skin (i.e., the
membrane has a pore asymmetry of 1:20,000). The water permeability of
the membrane is 0.30 cm/min psi.
Figure IV is a scanning electron microscope photomicrograph of a prior
art Millipore ultrafilter PSED, also having a molecular weight cut-off
of 25,000. This top surface is magnified 100,000 times and the bottom
surface is magnified 20,000 times. The pore asymmetry is 1:1. Its water
permeability is 0.01 cm/min psi.
A comparison of Figures III and IV illustrate the highly asymmetric
character of membranes produced according to the presently claimed
invention vis-a-vis prior art membranes.
The membranes of the present invention are integral membranes having a
skin which comprises generally less than about 5, typically from about
1 to about 5, and preferably from about I to about 3% of the thickness
of the membrane. The remaining part of the membrane is the asymmetric
support. There is a relatively sharp boundary between the skin layer
and the asymmetric support layer although both layers form an integral
membrane. There is a gradual change in pore size from the skin of the
membrane to the opposite face of the membrane. The gradient in pore
size in the asymmetric support is substantially logarithmic with
respect to thickness, increasing gradually from the barrier skin.
The water permeability, p, of the membranes of the present invention is
substantially linearly related to its retentive pore size, r, by the
equation p=K/r where K is the constant and for the membranes of the
present invention K is generally greater than 1.5 x 10 7cm /min psi,
typically greater than about 2 x /min psi, and preferably greater than
about 3 x 10 7cm /min psi. The coefficient of variation (i.e., the
standard deviation divided by the mean value) for the membranes of this
invention are typically between 30 and 50%. For example, a 4#a'
coefficient of variation is obtained using a 0.2 micron rated
polysulfone membrane.
The pore size distribution (i.e., the ratio of the maximum pore size to
the average pore size) within the skin of the- membranes of the present
invention is generally less than about 5, typically less than about 3,
and preferably less than about 2.
The membranes of the present invention have improved physical
properties over prior art membranes. For example, a 0.2 micron
polysulfone membrane produced according to the process of the 2 present
invention has a tensile strength of 30 kg/cm an elongation at break of
24% (see Example V) in comparison to an elongation of about 5% with
respect to 0.2 micron prior art membranes. These improved physical
properties are important in most membrane applications but are of
particular importance in medical applications where any membrane
rupture due to insufficient elongation could be critical.
The principles, preferred embodiments, and modes of operation of the
present invention have been described in the forgoing specification.
The invention which is intended to be protected herein, however, is not
to be construed as limited to the particular forms disclosed, since
these are to be regarded as illustrative rather than restrictive.
Variations and charles may be made by those skilled in this art without
departing from the spirit of the invention.