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RELATED APPLICATIONS
This application is related to copending U.S. application Ser. No. 314,307,
filed Oct. 23, 1981, now U.S. Pat. No. 4,473,474, which is a
continuation-in-part of U.S. application Ser. No. 201,366, filed Oct. 27,
1980 (now abandoned), both entitled "Charge Modified Microporous Membrane,
Process for Charge Modifying Said Membrane and Process for Filtration of
Fluid", to Ostreicher.
This application is also related to copending U.S. application Ser. No.
268,543, filed May 29, 1981, now U.S. Pat. No. 4,473,475, entitled "Charge
Modified Microporous Membrane, Process for Charge Modifying Said Membrane,
and Process for Filtration of Fluid", to Barnes, Jr. et al.
This application is further related to copending U.S. application Ser. No.
566,764, filed Dec. 29, 1983, now U.S. Pat. No. 4,604,208, entitled
"Anionic Charge Modified Microporous Membrane, Process for Charge
Modifying Said Microporous Membrane and Filtration of Fluid", to Chu et
al.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to microporous membranes, and more particularly to
surface modified microporous membranes suitable for the filtration of
aqueous fluids, such as biological liquids.
2. Prior Art
Microporous membranes are well known in the art. For example, U.S. Pat. No.
3,876,738 to Marinaccio et al. (1975) describes a process for preparing a
microporous membrane, for example, by quenching a solution of a film
forming polymer in a non-solvent system for the polymer. European patent
application No. 0 005 536 to Pall (1979) describes a similar process.
Commercially available microporous membranes, for example, made of nylon,
are available from Pall Corporation, Glen Cove, N. Y. under the trademark
"ULTIPOR N.sub.66 ". Such membranes are advertised as useful for the
sterile filtration of pharmaceuticals, e.g. removal of microorganisms.
Various studies in recent years, in particular Wallhausser, Journal of
Parenteral Drug Association, Jun., 1979, Vol. 33, #3, pp. 156-170, and
Howard et al, Journal of the Parenteral Drug Association, Mar.-Apr., 1980,
Volume 34, #2, pp. 94-102, have reported the phenomena of bacterial
breakthrough in filtration media, in spite of the fact that the media had
a low micrometer rating. For example, commercially available membrane
filters for bacterial removal are typically rated as having an effective
micrometer rating for the microreticulate membranes structure of 0.2
micrometers or less, yet such membrane typically have only a 0.357
effective micrometer rating for spherical contaminant particles, even when
rated as absolute for Ps. diminuta, the conventional test for bacterial
retention. This problem of passage of a few microorganisms under certain
conditions has been rendered more severe as the medical uses of filter
membranes has increased.
One method of addressing this problem is to prepare a tighter filter having
a sufficiently small effective pore dimension to capture microorganisms,
etc., by mechanical sieving. Such microporous membranes of 0.1 micrometer
rating or less may be readily prepared but flow rates at conventional
pressure drops are prohibitively low. Increasing the pressure drop to
provide the desired flow rate is not generally feasible because pressure
drop is an inverse function of the fourth power of pore diameter.
It has long been recognized that adsorptive effects can enhance the capture
of particulate contaminants. For example, Wenk, "Electrokinetic and
Chemical Aspects of Water Filtration", Filtration and Separation, May/Jun.
1974, indicates that surfactants, pH, and ionic strength may be used in
various ways to improve the efficiency of a filter by modifying the charge
characteristics of either the suspension, filter or both.
It has also been suggested that adsorptive sequestration (particle capture
within pore channels), may sometimes be more important in sterile
filtration than bubble point characterization of internal geometry
(representing the "largest pore"). See, e.g., Tanny et al, Journal of the
Parenteral Drug Association, Nov.-Dec. 1978, Vol. 21, #6, pp. 258-267 and
Jan.-Feb., 1979, Vol. 33, #1, pp. 40-51 and Lukaszewicz et al, Id.,
Jul.-Aug., 1979, Vol. 33, #4, pp. 187-194.
Pall et al, Colloids and Surfaces 1 (1980), pp. 235-256, indicates that if
the zeta potential of the pore walls of a membrane, e.g. nylon 66, and of
the particles are both low, or if they are oppositely charged, the
particle will tend to adhere to the pore walls, and the result will be
removal of particles smaller than the pores of the filter. Pall et al
suggest the use of membranes of substantially smaller pore size to
increase the probability of obtaining microbial sterility in filtering
fluids.
Zierdt, Applied and Environmental Microbiology, Dec. 1979, pp. 1166-1172,
found a strong adherence by bacteria, yeast, erythrocytes, leukocytes,
platelets, spores and polystyrene spheres to membrane materials during
filtration through membranes with pore-size diameters much larger than the
particles themselves. Zierdt found that cellulose membranes adsorbed more
bacteria, blood cells and other particles than did polycarbonate filters.
Of lesser adsorptive capacity were vinyl acetate, nylon, acrylic and
Teflon membranes. Zierdt additionally found that solvent cast membrane
filter materials, e.g. nylon, had strong surface charges, whereas ordinary
fibrous cellulose materials which are not solvent cast do not.
Attempts to increase the short life of filter media due to pore blockage
and enhance flow rates through filter media having small pores have been
made by charge modifying the media by various means to enhance capture
potential of the filter. For example, U.S. Pat. Nos. 4,007,113 and
4,007,114 to Ostreicher, describe the use of a melamine formaldehyde
cationic colloid to charge modify fibrous and particulate filter elements;
U.S. Pat. No. 4,305,782, to Ostreicher et al describes the use of an
inorganic cationic colloidal silica to charge modify such elements; and
U.S. Ser. No. 164,797, filed Jun. 30, 1980, to Ostreicher et al, describes
the use of a polyamido-polyamine epichlorhydrin cationic resin to charge
modify such filter elements. None of these references teaches or suggests
charge modifying a synthetic organic polymeric microporous membrane, nor
do any of the filtration media described therein, e.g. fiber and/or
particulate, provide the advantages of such a membrane.
Similarly, U.S. Pat. Nos. 3,242,073 (1966) and 3,352,424 (1967) to Guebert
et al, describe removal of micro-organisms from fluids by passage through
a filter medium of conventional anionic type filter aid, e.g. diatomaceous
earth, paper filter pulp, fullers earth, charcoal, etc., having an
adsorbed cationic, organic, polyelectrolyte coating. The coated filter aid
media is said to possess numerous cationic sites which are freely
available to attract and hold particles bearing a negative surface charge.
U.S. Pat. No. 4,178,438 to Hasset et al (1979) describes a process for the
purification of industrial effluent using cationically modified cellulose
containing material, e.g., bleached or unbleached pine sulphite cellulose,
kraft sulphate cellulose, paper, cardboard products, textiles fibers made
of cotton, rayon staple, jute, woodfibers, etc. The cationic substituent
is bonded to the cellulose via a grouping --O--CH.sub.2 --N--, where the
nitrogen belongs to an amide group of the cationic part and the oxygen to
the cellulose part.
There are numerous references which describe the treatment of porous
membranes for various objects. U.S. Pat. No. 3,556,305 to Shorr (1971)
describes a tripartite membrane for use in reverse osmosis comprising an
anisotropic porous substrate, an ultra-thin adhesive layer over the porous
substrate, and a thin diffusive membrane formed over the adhesive layer
and bound to the substrate by the adhesive layer. Such anisotropic porous
membranes are distinguished from isotropic, homogeneous membrane
structures used for microfiltration whose flow and retention properties
are independent of flow direction and which do not function properly when
utilized in the invention of Shorr.
U.S. Pat. No. 3,556,992 to Massuco (1971) describes another anisotropic
ultra-filtration membrane having thereon an adhering coating of
irreversibly compressed gel.
U.S. Pat. No. 3,808,305 to Gregor (1974) describes a charged membrane of
macroscopic homogeneity prepared by providing a solution containing a
matrix polymer, polyelectrolytes (for charge) and a crosslinking agent.
The solvent is evaporated from a cast film which is then chemically
cross-linked. The membranes are used for ultrafiltration.
U.S. Pat. Nos. 3,944,485 (1976) and 4,045,352 (1977) to Rembaum et al
describe ion exchange hollow fibers produced by introducing into the wall
of the pre-formed fiber, polymerizable liquid monomers which are then
polymerized to form solid, insoluble, ion exchange resin particles
embedded within the wall of the fiber. The treated fibers are useful as
membranes in water treatment, dialysis and generally to separate ionic
solutions. See also U.S. Pat. No. 4,014,798 to Rembaum (1977).
U.S. Pat. No. 4,005,012 to Wrasidlo (1977) describes a process for
producing a semi-permeable anisotropic membrane useful in reverse osmosis
processes. The membranes are prepared by forming a polymeric ultra-thin
film, possessing semi-permeable properties by contacting an amine modified
polyepihalohydrin with a polyfunctional agent and depositing this film on
the external surface of a microporous substrate. Preferred semi-permeable
membranes are polysulfone, polystyrene, cellulose butyrate, cellulose
nitrate and cellulose acetate.
U.S. Pat. No. 4,125,462 to Latty (1978) describes a coated semi-permeable
reverse osmosis membrane having an external layer or coating of a cationic
polyelectrolyte preferably poly(vinylimidazoline) in the bi-sulfate form.
U.S. Pat. No. 4,214,020 to Ward et al (1980) describes a novel method of
coating the exteriors of a bundle of hollow-fiber semi-permeable membranes
for use in fluid separations. Typical polymers coated are polysulfones,
polystyrenes, polycarbonates, cellulosic polymers, polyamides and
polyimides. Numerous depositable materials are listed, see col. 10, lines
55 - col. 12, for example, poly(epichlorhydrin) or polyamides.
U.S. Pat. No. 4,239,714 to Sparks et al (1980) describes a method of
modifying the pore size distribution of a separation media to provide it
with a sharp upper cut-off of a preselected molecular size. This is
accomplished by effectively blocking the entrances to all of the pores
larger than a preselected desired cut-off size, but leaving unchanged the
smaller pores. The separation media may be in the form of polymeric
membranes, e.g. cellulose acetate, cellulose nitrate, polycarbonates,
polyolefins, polyacrylics, and polysulfones. The pores are filled with a
volatile liquid which is evaporated to form voids at the pore entrances
and a concentrated solution of a crosslinkable or polymerizable pore
blocking agent, such as protein, enzyme, or polymeric materials is then
applied to the surface of the membrane.
U.S. Pat. No. 4,250,029 to Kiser et al (1981) describes coated membranes
having two or more external coatings of polyelectrolytes with at least one
oppositely charged adjacent pair separated by a layer of material which is
substantially charge neutralized. Kiser et al is primarily directed to the
use of charged membranes to repel ions and thereby prevent passage through
the membrane pores. The coated membranes are described as ordinary
semi-permeable membranes used for ultrafiltration, reverse osmosis,
electrodialysis or other filtration processes. A microscopic observation
of the coated membranes shows microscopic hills and valleys of
polyelectrolyte coating formed on the original external smooth skin of the
membrane. The membranes are particularly useful for deionizing aqueous
solutions. Preferred membranes are organic polymeric membranes used for
ultrafiltration and reverse osmosis processes, e.g., polyimide,
polysulfone, aliphatic and aromatic nylons, polyamides, etc. Preferred
membranes are anisotropic hollow fiber membranes having an apparent pore
diameter of from about 21 to about 480 angstroms.
Charge modified microporous filter membranes are disclosed in U.S. Ser. No.
358,822 of Ostreicher, filed May 9, 1973, now abandoned (corresponding to
Japanese Pat. No. 923,649 and French Pat. No. 74 15733). As disclosed
therein, an isotropic cellulose mixed ester membrane, was treated with a
cationic colloidal melamine-formaldehyde resin to provide charge
functionality. The membrane achieved only marginal charge modification.
Additionally, the membrane was discolored and embrittled by the treatment,
extractables exceeded desirable limits for certain critical applications,
and the membrane was not thermally sanitizable or sterilizable. Ostreicher
also suggests such treatment for the nylon membranes prepared by the
methods described in U.S. Pat. No. 3,783,894 to Lovell (1957) and U.S.
Pat. No. 3,408,315 to Paine (1968). It has been demonstrated that nylon
microporous membranes treated according to said Ostreicher reference would
also demonstrate marginal charge modification, high extractables and/or
inability to be thermally sanitizable or sterilizable.
The aforesaid Ostreicher U.S. Ser. No. 314,307 (published as PCT 0050804 on
May 5, 1982) generally describes a novel cationic charge modified
microporous membrane comprising a hydrophilic organic polymeric
microporous membrane and a charge modifying amount of a primary cationic
charge modifying agent bonded to substantially all of the internal
microstructure of the membrane. The primary charge modifying agent is a
water-soluble organic polymer having a molecular weight greater than about
1,000 wherein each monomer thereof has at least one epoxide group capable
of bonding to the surface of the membrane and at least one tertiary amine
or quaternary ammonium group. Preferably, a portion of the epoxy groups on
the organic polymer are bonded to a secondary charge modifying agent
selected from the group consisting of:
(i) aliphatic amines having at least one primary amino or at least two
secondary amino groups; and
(ii) aliphatic amines having at least one secondary amino and a carboxyl or
hydroxyl substituent.
The membrane is made by a process for cationically charge modifying a
hydrophilic organic polymeric microporous membrane by applying to the
membrane the aforesaid charge modifying agents, preferably by contacting
the membrane with aqueous solutions of the charge modifying agents. The
preferred microporous membrane is nylon, the preferred primary and
secondary charge modifying agents are, respectively, polyamido-polyamine
epichlorohydrin and tetraethylene pentamine. The charge modified
microporous membrane may be used for the filtration of fluids,
particularly parenteral or biological liquids. The membrane has low
extractables and is sanitizable or sterilizable.
The aforesaid Chu et al Ser. No. 566,764 generally describes a novel
anionic charge modified microporous membrane comprising a hydrophilic
organic polymeric microporous membrane and a charge modifying amount of
anionic charge modifying agent bonded to substantially all of the membrane
microstructure. The anionic charge modifying agent is preferably a
water-soluble polymer having anionic functional groups, e.g. carboxyl,
phosphonous, phosphonic and sulfonic groups. The charged membrane is made
by a process of applying the anionic charge modifying agent to the
membrane, preferably by contacting the membrane with aqueous solutions of
the charge modifying agent.
The just described applications describe a comparatively complex treatment
of a preformed membrane requiring treatment, rinse and drying steps which
involve complicated equipment and expensive capital investment.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of this invention to provide a process for surface
modifying a hydrophilic organic polymeric microporous membrane so as to
provide a novel surface modified microporous membrane, particularly
suitable for the microfiltration of biological or parenteral liquids.
It is another object of this invention to provide an isotropic, surface
modified microporous membrane which preferably has low extractables
suitable for the microfiltration of biological or parenteral liquids.
It is yet another object of this invention to prepare a sanitizable or
sterilizable microporous membrane for the efficient removal of bacteria,
viruses and pyrogen from contaminated liquids.
A still further object of this invention is to provide a process for
enhancing the filtration, adsorptive and/or capacity of microporous
membranes without affecting the internal microreticulate structure.
It is still a further object of this invention to provide a process for
producing a microporous membrane capable of capturing anionic or cationic
particulate contaminant of a size smaller than the effective pore size of
the membrane.
These and other objects of this invention are attained by a process for
surface modifying a hydrophilic organic polymeric microporous membrane by
forming the membrane from a composition containing surface modifying
agents. The preferred microporous membrane is nylon, the preferred surface
modifying agents are polyamido-polyamine epichlorohydrin, ethylene diamine
tetraacetic acid, carbon, silica and other chromatographic additives, poly
(styrene sulfonic acid) and poly (acrylic acid).
The surface modified microporous membrane produced by this invention may be
used for the microfiltration of fluids, particularly parenteral or
biological liquids.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a time vs. transmittance graph of membranes described in Example
V.
DETAILED DESCRIPTION OF THE INVENTION
The process of this invention produces a hydrophilic surface modified
organic polymeric microporous membrane.
By the use of the term "microporous membrane" as used herein, it is meant a
skinless ("symmetric"), isotropic or anisotropic porous membrane having a
pore size of at least 0.05 microns or larger, or an initial bubble point
(IBP), as that term is used herein, in water of less than 120 psi. A
maximum pore size useful for this invention is about 1.2 micron or an IBP
of greater than about 10 psi. By "isotropic" it is meant that the pore
structure is substantially the same throughout the cross-sectional
structure of the membrane. By "anisotropic" is meant that the pore size
differs from one surface to the other. There are a number of commercially
available membranes not encompassed by the term "microporous membrane" or
"microfiltration membrane" such as those having one side formed with a
very light thin skin layer (skinned, i.e. asymmetric) which is supported
by a much more porous open structure which are typically used for reverse
osmosis, ultrafiltration and dialysis. Thus, by the term "microporous
membrane" or "microfiltration membrane" it is meant membranes suitable for
the removal of suspended solids and particulates from fluids and which do
not function as ultrafiltration or reverse osmosis membranes but which may
have adsorptive and/or sequestration capacity.
By "surface modified microporous membranes" it is meant microporous
membranes which provide surface adsorption and/or sequestration effects in
addition to the microfiltration effects of the membranes per se. By
adsorptive surface, it is meant a surface that has controlled molecular
geometry and/or surface functionality that allows species to be attached
to the surface by means of ionic, covalent, hydrogen and/or Van Der Walls
bonding and/or molecular geometric effects, e.g. ionic exchange, affinity,
frontal, size exclusion and the like.
By the use of the term "hydrophilic" in describing the microporous
membrane, it is meant a membrane which adsorbs or absorbs water.
Generally, such hydrophilicity is produced by a sufficient amount of
hydroxyl (OH--), carboxyl (--COOH), amino (--NH.sub.2),
##STR1##
and/or similar functional groups on the surface of the membrane. Such
groups assist in the adsorption and/or absorption of the water onto the
membrane. Such hydrophilicity of the membrane and internal microstructure
of the surface modified membrane of this invention is preferred in order
to render the membrane more useful for the treatment of aqueous fluids.
Preferred microporous membranes are produced from nylon. The term "nylon"
is intended to embrace film forming polyamide resins including copolymers
and terpolymers which include the recurring amido grouping.
While, generally, the various nylon or polyamide resins are all copolymers
of a diamine and a dicarboxylic acid, or homopolymers of a lactam of an
amino acid, they vary widely in crystallinity or solid structure, melting
point, and other physical properties. Preferred nylons for use in this
invention are copolymers of hexamethylene diamine and adipic acid (nylon
66), copolymers of hexamethylene diamine and sebacic acid (nylon 610) and
homopolymers of poly-o-caprolactam (nylon 6).
Alternatively, these preferred polyamide resins have a ratio of methylene
(CH.sub.2) to amide (NHCO) groups within the range about 5:1 to about 8:1,
most preferably about 5:1 to about 7:1. Nylon 6 and nylon 66 each have a
ratio of 6:1, whereas nylon 610 has a ratio of 8:1.
The nylon polymers are available in a wide variety of grades, which vary
appreciably with respect to molecular weight, within the range from about
15,000 to about 42,000 and in other characteristics.
The highly preferred species of the units composing the polymer chain is
polyhexamethylene adipamide, i.e. nylon 66, and molecular weights in the
range above about 30,000 are preferred.
To the extent that commercially available polymers contain additives such
as antioxidants and the like, such additives are included within the term
"polymer" as used herein.
The membrane substrates can be produced by modifying the method disclosed
in U.S. Pat. No. 3,876,738 to Marinaccio et al or described in European
patent application No. 0 005 536 to Pall. The entire disclosures of both
of these references are incorporated herein by reference.
The Marinaccio et al process for producing membrane develops a unique fine
internal microstructure through the quench technique described therein,
offering a superior substrate for filtration. Broadly, Marinaccio et al
produces microporous films by casting or extruding a solution of a
film-forming polymer in a solvent system into a quenching bath comprised
of a non-solvent system for the polymer. Although the non-solvent system
may comprise only a non-solvent, the solvent system may consist of any
combination of materials provided the resultant non-solvent system is
capable of setting a film and is not deleterious to the formed film. For
example, the non-solvent system may consist of materials such as
water/salt, alcohol/salt or other solvent-chemical mixtures. The
Marinaccio et al process is especially effective for producing nylon
films. More specifically, the general steps of the process involve first
forming a solution of the film-forming polymer, casting the solution to
form a film and quenching the film in a bath which includes a non-solvent
for the polymer.
The nylon solutions which can be used in the Marinaccio et al process
include solutions of certain nylons in various solvents, such as lower
alkanols, e.g., methanol, ethanol and butanol, including mixtures thereof.
It is known that other nylons will dissolve in solutions of acids in which
it behaves as a polyelectrolyte and such solutions are useful.
Representative acids include, for example, formic acid, citric acid,
acetic acid, maleic acid and similar acids which react with nylons through
protonation of nitrogen in the amide group characteristic of nylon.
The nylon solutions after formation are diluted with non-solvent for nylon
and the non-solvent employed is miscible with the nylon solution. Dilution
with non-solvent may, according to Marinaccio et al, be effected up to the
point of incipient precipitation of the nylon. The non-solvents are
selected on the basis of the nylon solvent utilized. For example, when
water-miscible nylon solvents are employed, water can be employed.
Generally, the non-solvent can be methyl formate, aqueous lower alcohols,
such as methanol and ethanol, polyols such as glycerol, glycols,
polyglycols and ethers and esters thereof, water and mixtures of such
compounds. Moreover, salts can also be used to control solution
properties.
The quenching bath may or may not be comprised of the same non-solvent
selected for preparation of the nylon solution and may also contain small
amounts of the solvent employed in the nylon solution. However, the ratio
of solvent to non-solvent is lower in the quenching bath than in the
polymer solution in order that the desired result be obtained. The
quenching bath may also include other non-solvents, e.g. water.
The formation of the polymer film can be accomplished by any of the
recognized methods familiar to the art. The preferred method is casting
using a knife edge which controls the thickness of the cast film. The
thickness of the film will be dictated by the intended use of the
microporous product. In general, the films will be cast at thicknesses in
the range of from about 1 mil to about 20 mils, preferably from about 1 to
about 10 mils.
Preferably, the polymer solution is cast and simultaneously quenched,
although it may be desirable to pass the cast film through a short air
evaporation zone prior to the quench bath. This latter technique is,
however, not preferred.
After the polymer solution is cast and quenched, it is removed from the
quench bath and preferably washed free of solvent and/or non-solvent.
Subsequently the film can be at least partially dried.
Pall's aforementioned European patent application No. 0 005 536 describes
another similar method for the conversion of polymer into microporous
membrane which may be used. Broadly, Pall provides a process for preparing
skinless hydrophilic alcohol-insoluble polyamide membranes by preparing a
solution of an alcohol-insoluble polyamide resin in a polyamide solvent.
Nucleation of the solution is induced by the controlled addition to the
solution of a non-solvent for the polyamide resin, under controlled
conditions of concentration, temperature, addition rate, and degree of
agitation to obtain a visible precipitate of polyamide resin particles
(which may or may not partially or completely redissolve) thereby forming
a casting solution.
The casting solution is then spread on a substrate to form a thin film. The
film is then contacted and diluted with a mixture of solvent and
nonsolvent liquids containing a substantial proportion of the solvent
liquid, but less than the proportion in the casting solution, thereby
precipitating polyamide resin from the casting solution in the form of a
thin skinless hydrophilic membrane. The resulting membrane is then washed
and dried.
In Pall's preferred embodiment of the process, the solvent for the
polyamide resin solution is formic acid and the nonsolvent is water. The
polyamide resin solution film is contacted with the nonsolvent by
immersing the film, carried on the substrate, in a bath of nonsolvent
comprising water containing a substantial proportion of formic acid.
The nylon membrane described in Marinaccio et al and Pall are characterized
by hydrophilic isotropic structure, having a high effective surface area
and a fine internal microstructure of controlled pore dimensions with
narrow pore size distribution and adequate pore volume. For example, a
representative 0.22 micrometer rated nylon 66 membrane (polyhexamethylene
adipamide) exhibits an initial bubble point (IBP) of about 45 to 50 psid,
a foam all over point (FAOP) of about 50 to 55 psid, provides a flow of
from 70 to 80 ml/min of water at 5 psid (47 mm. diameter discs), has a
surface area (BET, nitrogen adsorption) of about 13 m.sup.2 /g and a
thickness of about 4.5 to 4.75 mils.
As will be apparent from the foregoing description, both the Marinaccio and
Pall processes involve the formation of a nylon polymer solution or dope
which is then diluted with a non-solvent, cast on a suitable substrate
surface and contacted with additional non-solvent to cause precipitation
of the polyamide resin from the dope solution in the form of a thin
skinless hydrophilic membrane. In the aforementioned pending Ostreicher et
al, Barnes et al and Chu et al applications, the resulting membrane is
charge modified by contacting the formed membrane with a charge modifying
amount of a charge modifying agent. In the present invention, the surface
modifying agent (which can be a cationic or anionic charge modifying
agent) is incorporated into the polymer solution or dope before the
membrane is precipitated. The membrane can thereafter be formed by the
casting technique described in Marinaccio et al and Pall or alternatively,
the dope can be introduced into the quenching bath of the non-solvent
under shear to produce fibers of the surface modified membrane which can
be formed into a sheet material similarly to the formation of paper from
fibers, e.g. as described in U.S. Pat. No. 4,309,247 to Hou et al (1982)
or made into hollow fibers to produce surface modified hollow fibers
similar to those produced in copending U.S. application Ser. No. 566,762
to Marinaccio, filed Dec. 29, 1983.
The surface modifying agent is bound to the internal microstructure,
preferably substantially all of the internal microstructure, of the
microporous membrane. By the use of the term "bound" it is meant that the
surface modifying agent is sufficiently attached to or incorporated into
the membrane so that it will not significantly extract under the intended
conditions of use. By the use of the term "substantially all of the
internal microstructure" as used herein, it is meant substantially all of
the external surface and internal pore surfaces. Typically by this is
meant the surfaces which are wetted by a fluid, e.g., water, passing
through the membrane or in which the membrane is immersed.
The term "surface modifying agent" means a compound, material or
composition which when bound to the membrane, alters its capacity to
remove a desired entity from a fluid being filtered and which is
compatible with the dope. By the use of the term "charge modifying agent",
it is meant a compound or composition that when bound to the microporous
filter membrane alters the "zeta potential" of the membrane (see Knight et
al, "Measuring the Electrokinetic Properties of Charged Filter Media,"
Filtration and Separation, pp. 30-34, Jan./Feb. 1981).
The cationic charge modifier is a compound or composition which is capable
of being bound to the membrane microstructure and provides a more positive
zeta potential to the membrane microstructure. Preferably, such cationic
charge modifier is a water-soluble compound having substituents capable of
binding to the membrane and substituents which are capable of producing a
more positive "zeta potential" in the use environment (e.g. aqueous) or
cationic functional groups. Most preferably, the agent may be a
water-soluble organic polymer capable of becoming a non-extractable
contituent of the membrane.
The cationic charge modifying agent can also be cross-linked to itself or
to the membrane polymer through a cross-linking agent, for example, an
aliphatic polyepoxide having a molecular weight of less than about 500.
The cationic charge modifying agent may have either a high or low charge
density, or anything between these extremes, however, high charge density
is preferred.
The preferred cationic charge modifier is selected from the class of
polyamido-polyamine epichlorohydrin cationic resins, in particular, those
described in the following U.S. Pat. Nos.:
2,926,116 to Keim;
2,926,154 to Keim;
3,224,986 to Butler et al;
3,311,594 to Earle, Jr.;
3,332,901 to Keim;
3,382,096 to Boardman; and
3,761,350 to Munjat et al.
The entire disclosures of all of these references are incorporated herein
by reference.
Broadly, these preferred charge modifiers (hereinafter "polyamido-polyamine
epichlorohydrin") are produced by reacting a long chain polyamide with
epichlorohydrin, i.e. 1-chloro-2,3 epoxypropane having the formula:
##STR2##
The polyamide may be derived from the reaction of a polyalkylene polyamine
and a saturated aliphatic dibasic carboxylic acid containing from about 3
to 10 carbon atoms. The polyamide produced is water-soluble and contains
the recurring groups:
--NH(C.sub.n H.sub.2n HN).sub.x --CORCO--
where n and x are each 2 or more and R is the divalent hydrocarbon radical
of the dicarboxylic acid. This polyamide is then reacted with
epichlorohydrin to form the preferred water-soluble charge modifiers used
in its invention.
The dicarboxylic acids which may be used in preparing the polyamides are
the saturated aliphatic dicarboxylic acids containing from 3 to 10 carbon
atoms each as malonic, succinic, glutaric, adipic, azelaic and the like.
Blends of two or more of the saturated carboxylic acids may also be used.
A variety of polyalkylene polyamines including polyethylene polyamines,
polypropylene polyamines, polybutylene polyamides and so on may be
employed. More specifically, the polyalkylene polyamines are polyamines
containing two primary amine groups and at least one secondary amine group
in which the nitrogen atoms are linked together by groups of the formula
--C.sub.n H.sub.2n --, where n is a small integer greater than unity and
the number of such groups in the molecule ranges from two up to about
eight. The nitrogen atoms may be attached to adjacent carbon atoms in the
group --C.sub.n H.sub.2n -- or to carbon atoms further apart, but not to
the same carbon atom. Polyamines such as diethylenetriamine,
triethylenetetramine, tetraethylene-pentamine, dipropylenetriamine, and
the like, and mixtures thereof may be used. Generally, these polyalkylene
polyamines have the general formula:
H.sub.2 [(C.sub.n H.sub.2n)NH].sub.y C.sub.n H.sub.2n NH.sub.2
wherein n is an integer of at least 2 and y is an integer of 1 to 7.
In carrying out the reaction of the polyalkylene polyamine with the acid,
it is preferred to use an amount of dicarboxylic acid sufficient to react
substantially completely with the primary amine groups of the polyalkylene
polyamine but insufficient to react with the secondary amine groups to any
substantial extent. The polyamide produced is then reacted with the
epichlorohydrin to form the preferred polyamido-polyamine epichlorohydrin
charge modifying agent. Typically, in the polyamide-epichlorohydrin
reaction it is preferred to use sufficient epichlorohydrin to convert all
of the secondary amine groups to tertiary amine groups, and/or quaternary
ammonium groups (including cyclic structures). Generally, however, from
about 0.5 mol to about 1.8 moles of epichlorohydrin for each secondary
amine group of the polyamide may be used.
The epichlorohydrin may also be reacted with a polyaminoureylene containing
tertiary amine nitrogens to produce the primary charge modifying agents
which may be utilized in this invention (see for example the
aforementioned Earle, Jr.).
Other suitable charge modifying agents of the foregoing type may be
produced by reacting a heterocyclic dicarboxylic acid with a diamine or
polyalkylene polyamine and reacting the resultant product with
epichlorohydrin (see for example the aforementioned Munjat et al.)
The polyamido-polyamine epichlorohydrin cationic resins are available
commercially as Polycup 172, 1884, 2002 or S2064 (Hercules); Cascamide
Resin pR-420 (Borden); or Nopcobond 35 (Nopco). Most preferably, the
polyamido-polyamine epichlorhydrin resin is Polycup 1884 or Hercules
R4308, wherein the charged nitrogen atom forms part of a heterocyclic
grouping and is bonded through methylene to a depending, reactive epoxide
group.
Each monomer group in R 4308 has the general formula:
##STR3##
Polycup 172, 2002 and 1884, on the other hand, have monomer groups of the
general formula:
##STR4##
wherein R is methyl or hydrogen (Polycup 172 and 2002, R.dbd.H; and
Polycup 1884, R.dbd.CH.sub.3).
A secondary charge modifying agent may be used to enhance the cationic
charge of the primary charge modifying agent and/or enhance the bonding of
the primary charge modifying agent. The secondary charge modifying agent
may be selected from the group consisting of:
(i) aliphatic amines having at least one primary amino or at least two
secondary amino groups; and
(ii) aliphatic amines having at least one secondary amine and a carboxyl or
hydroxyl substituent.
Preferably, the secondary charge modifying agent is a polyamide having the
formula:
##STR5##
wherein R.sub.1 and R.sub.2 are alkyl of 1 to 4 carbon atoms and x is an
integer from 0 to 4. Preferably, R.sub.1 and R.sub.2 are both ethyl.
Preferred polyamines are:
______________________________________
Ethylene diamine
H.sub.2 N--(CH.sub.2).sub.2 --NH.sub.2 --NH.sub.2
Diethylenetri-
H.sub.2 N--(CH.sub.2).sub.2 --NH--(CH.sub.2).sub.2 --NH.sub.2
amine
Triethylenete-
H.sub.2 N--(CH.sub.2 --CH.sub.2 --NH).sub.2 --CH.sub.2
--CH.sub.2 --NH.sub.2
tramine
Tetraethylenepen-
H.sub.2 N--(CH.sub.2 --CH.sub.2 --NH).sub.3 --CH.sub.2
--CH.sub.2 --NH.sub.2
tamine
______________________________________
The highly preferred polyamine is tetraethylene pentamine.
Alternatively, aliphatic amines used in this invention may have at least
one secondary amine and a carboxyl or hydroxyl substituent. Exemplary of
such aliphatic amines are gamma-amino-butyric acid (H.sub.2 NCH.sub.2
CH.sub.2 CH.sub.2 COOH) and 2-amino-ethanol (H.sub.2 NCH.sub.2 CH.sub.2
OH).
The secondary charge modifying agent is bonded to the microporous membrane
by bonding to a portion of the epoxide substituents of the polymeric
primary charge modifying agent.
The amount of primary and secondary cationic charge modifying agent
utilized is an amount sufficient to enhance the electropositive capture
potential of the microporous membrane. Such an amount is highly dependent
on the specific charge modifying gents utilized. For general guidance,
however, it has been found that a weight ratio of primary to secondary
charge modifying agent of from about 2:1 to about 500:1, preferably from
about 25:1 to about 75:1 is generally sufficient.
In another embodiment of the present invention, the foregoing "secondary"
charge modifying agent can be used as the charge modifying agent by the
cojoint employment of an aliphatic polyepoxide crosslinking agent having a
molecular weight of less than about 500. Preferably, the polyepoxide is a
di- or tri- epoxide having a molecular weight of from about 146 to about
300. Such polyepoxides have viscosities (undiluted) of less than about 200
centipoises at 25.degree. C. Due to the necessity of the epoxide to act as
a crosslinking agent, monoepoxides, e.g. glycidyl ethers, are unsuitable.
Similarly, it is theorized that a polyepoxide offering greater than three
epoxy groups offers no benefit and in fact may limit the coupling
reactions of the polyepoxide by steric hindrance. Additionally, the
presence of unreacted epoxide groups in the cationically charge modified
microporous membrane may be undesirable in the finished product.
Highly preferred polyepoxides have the formula:
##STR6##
wherein R is an alkyl of 1 to 6 carbon atoms and n is from 2 to 3. The
limitation that the number of carbon atoms in the non-epoxide portion
--(R)-- be less than 6 is so that the polyepoxide will be soluble in water
or ethanol-water mixtures, e.g. up to 20% ethanol. While higher carbon
content materials are functionally suitable, their application would
involve the use of polar organic solvents with resulting problems in
toxicity, flammability and vapor emissions.
The anionic charge modifying agent is a compound or composition which is
capable of bonding to the membrane microstructure without substantial pore
size reduction or pore blockage and provides an anionic charge or negative
zeta potential to the membrane microstructure. Preferably, such anionic
charge modifier is a water-soluble compound having substituents capable of
binding to the membrane and substituents which are capable of producing a
more negative "zeta potential" in the use environment (e.g. aqueous) or
anionic functional groups.
Preferred anionic functional groups may be carboxyl, phosphonous,
phosphonic and sulfonic. Preferably, the anionic charge modifying agent
may be a water-soluble organic polymer or polyelectrolyte having a
molecular weight greater than about 2,000 and less than about 500,000 and
capable of becoming a non-extractable constituent of the membrane.
The anionic charge modifying agent may have either a high or low charge
density, or anything between these extremes, however high charge density
is preferred. Specific preferred anionic charge modifying agents useful
herein are poly (styrene sulfonic) acid, poly (toluene sulfonic) acid,
poly (vinyl sulfonic) acid and poly (acrylic) acid. Other anionic charge
modifying agents are poly (methacrylic acid), poly (itaconic acid),
hydrolyzed poly (styrene/maleic anhydride) and poly (vinyl phosphonic
acid). Additionally, the alkali and alkaline earth metal salts of all of
the foregoing may be utilized.
Highly preferred anionic charge modifying agents are poly (styrene
sulfonic) acids having a molecular weight between 2,000 and 300,000, and
poly (acrylic acid) having a molecular weight between 2,000 and 300,000.
The anionic charge modifying agent may also be crosslinked to the
microporous membrane structure or itself in the same manner as the
cationic agents using the same aliphatic polyepoxide cross-linking agent
having a molecular weight of less than about 500. In addition to the
preferred polyepoxides described above, certain diglycidyl ethers of
aliphatic diols,
##STR7##
may be used. Examples are 1,2-ethanediol, 1,3-propanediol, and
1,4-butanediol. The preferred diglycidyl ether of 1,4-butanediol is
commercially available from Ciba-Geigy, Inc. as RD-2 and from Celanese
Corp. as Epi-Rez 5022 and Polyscience.
Other higher carbon diglycidyl ethers may be used as the polyepoxide
cross-linking | | |
