Note: Descriptions are shown in the official language in which they were submitted.
,82S65
_ELATED APPLICATIONS
This application is related to U.S. Patent
No. 4,473,474 issued September 25, 1984, 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
United Scates Patent No. 4,473,475 issued September
25, 1984, enti-tled "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 rela-ted
to U.S. Patent No. 4,604,208 issued August 5,
:L986, ent:Ltlecl "Anionic Charge ModiEied Microporous
Membrarle, Process Eor Char~e ModlEying Said Micro-
porous Membrane and F:L:Ltratlon of Fluid", to
Chu eL al.
~1~
2565
BACKGROUND OF T~E I~VENTION
1. Field of the Invention
This invention relates to microporous membranes,
and more particularly to surface modified microporous mem-
branes 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. Patent No. 3,876,738 to Marinaccio et al.
(1975) describes a process for preparing a microporous mem-
brane, for e~ample, by quenching a solution of a film forming
polymer in a non-solvent system for the polymer. European
Patent Application 0 005 536 to Pall (1979) describes a
similar process.
Commercially available microporous membranes, for
example, made of nylon, are available ~rom Pall Corporation,
Glen Cove, New York under the trademark "ULTIPOR N66". Such
membranes are advertlsed as useful for the sterile filtration
of pnarmaceuticals, e.~. removal of microorganisms.
Various studies in recent years, in particular Wall~
hausser, Journal of Parenteral Drug Association, June, 1979,
Vol. 33, #~, pp. 156-170, and Howard et al, Journal of the
Parenteral Dru~ Association, March-April, 1930, Volume 34,
~2, p~. 94-102, have reported the phenomena of bacterial
breakthro-l~h in filtration media, in spite of the fact that
the media had a low micrometer rating. For example, commer-
clally available membrane filters for bacterial removal are
typically rated as havin~ an effective micrometer rating for
~he microre~iculate membranes structure of 0.2 micrometers
or less, yet such membranestypically have only a 0.357 effec-
tlve micrometer rating for spherical contaminant particles,
even ~vhen rated as absolute for Ps. diminuta~ the conven-
tlonal tes~ for bacterial retention. This-problem of pas-
sa~e of a few microor~anisms under certain conditions has
been rendered more severe as the medical uses of filter
I~ ~,v~
membranes ~ increased.
56~
Ona method of addressing this problem is to prepare
a tlghter fllter having a sufficiently small effective pore
dimension to capture microorganisms, etc., by mechanical
sievin~. Such microporous membranes of 0.1 micrometer rating
or less may be readily prepared but flow rates at conventional
~ressure drops are prohibitively low. Increasing the pressure
drop to provide the desired flow rate is not generally feasi-
ble because pressure drop is an inverse function of the
fourth power of pore diameter.
It has lon~ 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/June 1974, indi-
cates that surfactants, pEI, and ionic strength may be used
in various ways to improve the efficiency of a filter by mo-
difying the charge characteristics of either the suspension,
fllter or both.
It has also been su~ested that adsorptive seques-
tration tparticlo capture within pore channels), may some-
times be more important in sterile filtration than bubble
poinc characterization of internal geometry (representing
tne "largest pore"). See, e.g., Tanny et al, Journal OI the
Parenteral Drug Assoclation, November-December 1978, Vol.
~1, #6, pp. 258-267 and January-February, 1979, Vol~ 33,
#1, pp. 40-51 and Lukaszewicz et al, Id., July-August, 1979,
Vol. 33, #4, Pl~- 187-194.
Pall et al, Colloids and Surfaces 1 (1980), pp.
~35-256, indicates that if the zeta potential of the pore
walls of a membrane, e.g. nylon 66, and of the particles are
~oth low, or if they are oppositely charged, the particles
wlll tend to adhere to the pore walls, and the result will
~e 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.
~2~3Z565
Zierdt, Applied and Environmental Microbiology, Dec~
1979, pp. 1166-1172, found a strong adherence by bacterla, 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.
~ierdt found that cellulose membranes adsorbed more bacteria, blood
cells and other particles than did polycarbonate fil-ters. Of
lesser adsorptive capacity were vinyl acetate, nylon, acrylic and
TeflonTM 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 Eilter 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~ Patents ~,007,113 and ~,007,11~ to Ostreicher, describe the
use oE a melamine Eormaldehyde cat:ionic colloid to charge modify
fibrous and particulate filter elements; U.S. Patent No. ~,305,782,
to Ostreicher et al describes the use oE an inorganic cationic
colloidal silica to charye modiEy ~uch elements. None oE-these
reEererlces teaches or suyyests charge modiEy:irlg a synthetic organic
po.lymer:ic microporous membrane, nor do any of the filtration media
described thereln, e.g. Eiber and/or particulate, provide the
advantayes oE such a membrane.
Similarly, U.S. Patent No. 3,2~2,073 (1966) and 3,352,42
(1967) to Guebert et al, describe removal of microorganisms 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 c~tionic, organic, polyelectrolyte coatin~. The
coated filter aid media is said to possess numerous cationic
sites which are freely available to attract an~ hold parti-
cles bearing a negative surface charge.
U.S. Patent No. 4,178,438 to Hasset et al (1979)
describes a process for the purification of industrial efflu-
ent using cationically modified cellulose containing material,
e.~., 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
~roupin~ -0-C~2~N- , where the nitrogen belongs to an alnide
~rou~ 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.
Patent No. 3,556,305 to Shorr tl9?1) describes a tripartite
nembrane for use in reverse osmosis comprising an anisotropic
porous substrate, an ultra-thin adhesive layer over the
porous substrate, and a thi~ di~usive membrane ~ormed over
the adhesive layer and bounct to the substrate by the adhesive
layer. ~uch anisotropic porous membranes are distinguished
from isotro~ic, homo~eneous membrane structures used for
microfiltration whose flow and retention properties are in
de~endent of flow direction and which do not function properly
~ u hs ~ Je l/
when ~i~ed in the invention o~ Shorr.
U,S. Patent No. 3,556,992 to ,Uassuco (1971) describes
another anisotropic ultra-filtration membrane having thereon
an adherinJ coatin~ of irreversibly compressed gel.
U.S. Patent No. 3,808,305 to Gregor (1974) describes
a charged membrane of macroscopic homogeneity prepared by pro-
viding a solution containing a matrix polymer, polyelectro-
lytes (for char~e) and a crosslinking agent. The solvent is
~,
-
i5
evaporated fron~ a cast film which is then chemically cross-
linked. The membranes are used for ultrafiltration.
U.S. Patent No. 3,944,485 (1976) and 4,045,352 (1977)
to Rembaum et al describe ion exchange hollow fibers produced
by introducin~ into the wall of the pre-formed fiber, poly-
merizable liquid monomers which are then polymerized to form
solid, insoluble, ion exchan~e resin particles embedded
within the wall of the fiber. The treated fibers are useful
as membranes in water treat~ent, dialysis and generally to
separate ionic solutions. See also U.S. Patent No. 4 ,014,798
to Hembaum (1977).
U.S. Patent No. 4,005,012 to Wrasidlo (1977) describes
a process for producin~ a semi-permeable anisotropic membrane
useful in reverse osmosis processes. The membranes are pre-
pared by formin~ a polymeric ultra-thin film, possessing semi-
permeable properties by contacting an amine modified polyepi-
halohydrin with a polyfunctional a~ent and depositing this
film on the external surface of à microporous substrate.
Preferred semi-permeable membranes are polysulfone, polysty-
rene, cellulose butyrate, ce~lulose nitrate and cellulose
acetate.
U.S. Pa~ent l`lo. 4,12$,~62 to Latty (1978) describes
a coated semi-permeable reverse osmosls membra!le having an
external layer or ¢oatin~ o~ a cationic polyelectrolyte pre-
ferably poly~vinylimidazoline) in the bi-sulfate form.
U.S. Patent .~o. 4,214,020 to Ward et al (1980) de-
scribes a novel method of coatin~ the exteriors of a bundle
of hollow-fiber semi-permeable membranes for use in fluid
se~arations. Typical polymers coated are polysulfones, poly-
styrenes, polycarbonates, cellulosic polymers, polyamides
and polyimides. Numerous depositable materials are listed,
see col. 10, lines 55 - col. 12, for example, poly(epichlor-
hydrin) or polyamides.
U.S. Patent No. 4,239,714 to Sparks et al (1980)
descri~es a method of modifyin" the pore size distribution
~l2~ S65
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 pre-
selected 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, poly-
carbonates, 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 cross-
linkable or polymerizable pore blocking agent, such as protein,
enzyme, or polymeric materials is then applied to the surface of
the membrane.
U.S. Pa-tent 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 ad]acent pair
separated by a layer oE material which is substantially charge
neutralized. Kiser et al is primarily directed to the use of
charged me~branes to repel ions and thereby prevent passage through
the membrane pores. The coated membranes are described as ordinary
semi-permeable memhranes used for ultrafiltratlon, reverse osmosis,
elec-trod:ialys:Ls or o-ther Eiltration processes~ A ~icroscopic
observatlon o.E-the coated membranes shows microscopic hills and
valleys oE polyelectrolyte coak.ing Eormed on the original external
smooth sk:Ln of the rnembrane. rl'he tnembranes are particularly useful
for deioni~iny a~leous solutions. Preferred membranes are organic
polymeric membranes used for ultrafiltration and reverse osmosis
processes, e.g., polyimide, polysulfone, aliphatic and aromatic
nylons, polyamides, etc. PreEerred membranes are anisotropic
hollow fiber membranes having an apparent pore diameter of from
about 21 to about 480 angstxoms.
Charge modified microporous filter membranes are
disclosed in Canadian Patent No. 1,044,537 of Ostreicher, issued
~L~82~;6~ii
December 19, 1978, (correspondlng -to Japanese Patent No. 923,649).
As disclosed therein, an isotropic cellulose mixed ester membrane,
was trea-ted with a cationic colloidal melamine-formaldehyde resin
to provide charge functionality. The mernbrane 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. Patent No. 2,783,894 to Lovell (1957)
and U.S. Patent 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
rnodification, high extractables and/or inability to be thermally
sanitizable or sterilizable.
The afo.resaid Ostreicher U.S. Patent No. 4,473,474
(publ:ished as European 0050804 on May 5, 1982) generally deseribes
a novel cationie charge modl~led mieroporous membrane comprising a
hyclroph:i:Lic orgarlie polymerlc i~icroporous membrane ancl a charge
modify:incJ amotmt o:E a prlmary catlonle eharge modi~ying agent
bonded to substantla:L:Ly alL o~ the lnterna:L mierostructure of the
memb.rane. ~`he prlmary chc~r-~e modl~yin~ agent ls a water-soluble
organle polymer havlng a molecular weight greater than about 1,000
whereln each monomer thereof has at least one epoxlde group eapable
of bondlng to the sur:~aee of the membrane and at least one ter-tia-cy
amine or cluaternary ammonium group. Preferably, a portion of the
epoxy groups on the organie 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
-" ~21~25~i~
ii) aliphatic amines having at least one secondary amino
and a carboxyl or hydroxyl substituent.
The membrane is made by a process for ca-tionically charge modifying
a hydrophilic organic polymeric microporous membrane by applying to
the membrane the aforesaid charge modifying agents, preferably by
contacting the men~rane 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 of biological
liquids. The membrane has low extractables and is sanitizable or
sterilizable.
The aforesaid Chu et al U.S. Patent No. 4,604,208
generally describes a novel anionic charge modified microporous
membrane comprising a hydrophilic organic polymeric microporous
membrane and a charge moclifying amount of anionie charge modifying
agent bonded to suhstantiaL:Ly aLl of ~he membrane microstructure.
'rhe cmion:ic charge modiEying agent is preferably a water-soluble
polymer having anionic f~tnctional groups, e.g. ccarboxyl, phos-
phonous, phosphonic and s~lLfonic groups. '~'he charged me.mbrane is
made by a process Oe applying the anion.ic charge modifying agent to
the membrane, preEerably by contacting the membrane with aclueous
solutions of the charge modifying agent.
The just described Patents describe a comparatively
complex treatment of a preformed membrane requiring treatment,
rinse and drying steps which involve complicated equipment and
expensive capital investment.
OBJECl'S AND SUMM~RY OF T~E INVENTION
It is an object of this invention to provide a process
for surface modifying a hydrophilic organic polymeric
325~iS
microporous membrane so as to provide a novel surface modi-
fied micro~orous membrane, particularly suitable for the
microfiltration of biolo~ical or parenteral liquids.
It is another object of this inventon to provide an
isotro~ic, surface modified microporous membrane which pre-
ferably has low extractables suitable for the microfiltration
of biolo~ical or parenteral liquids.
I~ is yet another object of this invention to pre-
pare a sanitizable or sterilizable microporous membrane ~or
the efficient removal of bacteria, viruses and pyrogen from
contaminated liquids.
A still further object of this invention is to pro-
vide a process for enhancing the filtration, adsorptive
and/or capacity of microporous membranes without affecting
the internal microreticulate structure,
I~ is still a further object o~ this invention to
provide a process for producin~ a microporous membrane cap-
a~le o~ ca~turin~ anionic or cationic particulate contaminant
of a size smaller than the e~ective pore size of the membrane.
These and other obJects of thls invention are attained
by a process for surEace rnodlfying a hydrophilic organic poly-
meric microporous membrane by ~orming the membrane from a
com~osition containin~ surface modifying a~ents. The pre-
~`erred microporous membrane is nylon, the preferred surface
modi~yin~ a~ents are ~olyamido-polyamine epichlorohydrin,
ethylene diamine tetraacetic acid, carbon, silica and other
chromato~ra~hic additives, poly (styrene sulfonic acid) and
~oly (acrylic acid).
The surface modified microporous membrane produced
by tnis invention may be used for the microfiltration of
fluids, particularly parenteral or biological liquids.
BRIEF DESCRIPTION OF THE FIGURES
Fi~ure 1 is a time vs. transmittance graph of mem-
~ranes described in Example V.
8256~
DETAlT~n DESCRIPTION OF THE INVENTION
The process of this invention produces a hydrophilic
surface modified organic polymeric microporous membrane.
~ the use of the term "microporous membrane" as used
herein is meant a skinless ("symmetric"), isotropic or anisotropic
porous membrane having a pore size of at least .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. mere are a number of
commerc:ially available membranes nat encompassed by the term
"microporous membrane" or "microfiltration membrane" such as those
having one side foxmed with a very light thin s]cin layer (skinned,
i.e. asymmetrlc) which is supported by a much more porous open
structure which are typically used for reverse osmosis, ultra-
Eiltratlon and dialysis. Thus, by the ~erm "microporous membrane"
or "microEiltratlon membrane" are mecmt membranes suitc~ble for the
removal oE suspencled soLids arld par-ticulates from Eluids and which
do not funct:ion as ultraEiltration or reverse osmosis membranes but
which mc~ have adsorptive and/or se~uestration capac:ity.
By "surface IwdiEied microporous membranes" are 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.
565
12
By the use of the term "hydrophilic" in describing
the microporous membrane, it is meant a membrane which ad-
sorbs or absorbs water. Generally, such hydrophilicity is
produced by a sufficient amount of hydroxyl (OH-), carboxyl
(-CO~H), amino (-NH2)~ (-C-NH-), and/or similar functional
O
~rou~s on the surface of the membrane. Such groups assist
in the adsor~tion 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 includin~ copolymers and terpolymers which
include the recurring amido groupin~.
While, ~ellerally, the various nylon or polyamide
resins are all copolymers o~ a diamine and a dicarbo~ylic
aaid, or homopolymers of a lactam o~ an amino acid, they
vary widely in crystalllnity or solid structure, melting
~oint, and other physical properties. Pre~erred nylons for
use in this invention are copolymers of hexamethylene dia-
mine and adipic acid (nylon 66), copolymers of hexamethylene
diamine a,nd sebacic acid (nylon 610) and homopolymers of
poly-o-caprolactam (nylon 6).
~ lternatively, these preferred polyamide resins
have a ratio of methylen~ (CH2) to amide (NHCO) groups
within the ran~e about 5:1 to about 8:1, most preferably
a~out 5:1 to about 7:1. Nylon 6 and nylon 66 each have a
ratio of 6:l, whereas nylon 610 has a ratio of 8:1.
The nylon polymers are available in a wide variety
of ~rades, which vary appreciably with respect to molecular
wel~ht, within the ran~e from about 15,000 to about 42,000
and in otner characteristics.
8.~S~;5
The highly preferred species of the unites 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. Pa-tent No. 3,876,738 to Marinaccio et al
or described in European Patent Application No. 0 005 536 to Pall.
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
eomprise only a non-solvent, the solvent system may eonsist of any
combination of materials provided the resultant non-solvent system
ls eapable o:E setting a film and is not cleleterious to the fo.rmed
Eilm. For exc~mple, the non~solvent system may consist of materials
sueh as water/salt, aleohol/salt or other solvent-chemical
mixtures. 'I'he Marinaceio et al proeess is espee:ially effeetive for
produeing nylon films. More speeifieally, the general steps of the
proeess involve first forming a solution of the :Eilm-forming
polymer, easting the solution to form a film and q~lenehing the film
in a bath whieh ineludes a non-solvent for the polymer.
The nylon solutions which can be used in the MaLinaccio
et al process include solutions of certain nylons in various
solvents, such as lower alkanols, e.g., methanol,
~ "," ,~
:
14
e~hanol and butanol, including mixtures thereof. It is known
that other nylons will dissolve in solutions of acids in which
f~SC)~ be)~
it behavco 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 formatel aqueous lower alcohols, such as methanol and
ethanol, polyols such as glycerol, glycols, polyglycols and
ethers and esters thereof, water and mixtures of such com-
pouncls. Moreover, salts can also be used to control solu-
tion ~ro~erties.
The ~luenchin~ bath Inay or may not be comprised of
the sa~e non-solvent selected for ~reparation of the nylon
solution and may also contain small amounts of the solvent
em~loyed in the nylon solution. However, the ratio of sol-
vent to non-solven~ is lower in the quenchin~ bath than in
the polymer solution in order thQt the desired result be
obtained. The quenchinO bath may also include other non-
solvents, e.g. water,
The formation of the polymer film can be accomplished
~y any of the reco~nized methods familiar to the art. The
~referred method is castin~ usin~ 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
~roduct. In genera1, the films will be cast at thicknesses
in the range of from about 1 mil to about 20 mils, prefer-
a~ly froln about 1 to about 10 mils.
5~S
Preferably, the polymer solution is cast and simul-
taneously 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 pre-
ferred.
- 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 con-
version of polymer into microporous membrane which may be
used. Broadly, Pall provides a process for preparing skin-
less hydrophilic alcohol-insoluble polyamide membranes by
~reparing a solution of an alcohol-insoluble polyamide resin
in a polyamide solvent. Nucleation of the solution is in-
duced by the controlled a~dition to the solution of a non-
solvent for the polyamide resin, under controlled conditions
of concentration, temperature, addition rate, and degree of
fl~itation to obtain a visible precipitate of polyamide resin
particles (which may or rnay not partially or completely re-
dissolve) thereby forming a castln~ solution.
The castin~ solution is then spread on a substrate
to form a thin film. The film is then contacted and diluted
with a mixtur~ of solvent and nonsolvent liquids corltaining
a substantial proportion of the solvent liquid, but less
than the proportion in the casting solution, thereby preci-
~ltating polyamide resin from the casting solution in the
form of a thin skinless hydrophilic membrane. The resulting
mem~rane 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 noasolvent is water. The polyamide resin solution film
is contacted with the nonsolvent by immersing the film, car-
ried on the substrate, in a bath of nonsolvent comprising
water containing a substantial proportion of formic acid.
2S6S`
16
rrhe nylon membranes described in Marinaccio et al and
Pall are characterized ~y hydrophilic isotropic structure, having a
high effective surface area and a fine internal microstructure of
controlled pore dimensions ~ith narrow pore size distribution and
adequate poxe 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 (F~OP) 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 m2/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,
cas-t 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 afo:rement:Loned Patents to Ostreicher et al, Barnes et al and
Chu et al, the resulting membrane is charge modified by contacting
the formed mel~brane with a charge modifying amount of a charge
mod:ifying agent. ~n the present invention, the surface modifying
agent (which can be a cationi.c or anionic charge modifying agent)
is incorpo.rated .into the polymer solution or dope before the
membrane is precipitated~ rrhe membrane can thereafter be fo.rmed 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. Patent 4,309,247 to Hou et al (1982) or made into hollow
fibers to produce surface m~dified hollow fibers.
2~3ZSÇ;~
17
The surface modifying agent is bound to the internal
microstructure, preferably substantially all of the internal
microstructure, of the microporous membrane. sy the use of the
term "hound" is meant that the surface modifying agent is
sufficiently attached to or incorporated into the membrane so that
it will not be significantly extracted under the intended
conditions of use. By the use of the term "substantially all of
the internal microstructure" as used herein, 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", is meant a compound or composition that
when hound to the microporous Eilter membrane alters the "zeta
potential" of the membrane (see Knight et al, "Measuring the
Electro]cinetic Properties of Charged Filter Media," Filtration and
Sepc~ration, pp. 30-3~, ;lc~n./Feb. 19~1).
l'he cati.onlc charge ~lodi;Eier is a compound or composition
which is capable of being bound to the membrane microstructure and
provides a more posit:lve zeta potential to the membrane micro-
structure. PreEerably, such cationic charge modifier is a
water-soluble compound having substituents capable of binding to
the membrane and substituen-ts which are capable of producing a more
positive "zeta potential" in the use enviromnent (e.g. aqueous) or
cationic functional groups. Most preferably, the agent may be a
water-soluble organic polymer capable of becoming a non-extractable
constituent of the membrane.
2~6~
18
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 charye 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. patents:
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.
Broadly, these preerred charge modifiers (hereinafter
"polyatnido-polyatnine epichlorohydrin") are produced by reacting a
loncJ chain polyamide w:Lth epichlorohydrin, i.e. 1 - chloro-2,3
epoxypropane having the Eormula:
,0_
CII2 CH CH2Cl.
The polyamide may be derived frorn 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 watex-soluble and contains the recurring groups:
-N~I(CnH2n~)X-CORCO-
56~
19
where n and x are each 2 or more and R is the divalent hydro-
carbon 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 poly-
ethylene polyamines, polypropylene polyamines, polybutylene
polyamides and so on may be employed. More specifically,
the polyalkylene polyamines are polyamines containing two
primary amine ~roups and at least one secondary amine group
in which the nitro~en atoms are linked together by groups of
the formula - C~H2n-l where n is a small integer greater
than unity and the number of such ~roups in the molecule
ranges from two up to about eight. The nitrogen atoms may
be attached to adjacent carbon atoms in the group -CnH2n_ or
to carbon ato~s further apart, hut not to the same carbon
atom. Polyamines suoh as diethylenetriamine, triethylene-
tetrarnine, tetraethylene-pentamine, dipropylenetriamine, and
the like, and mixtures thereo~ may be used. Generally,
these polyalkylene polyamines have the general formula:
H2[(CnH2n)NH]yCn~l2nN~2
wherein n is an integer of at least 2 and y is an integer of
1 to 7.
In carryin~ 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
witn the primary amine groups of the polyalkylene polyamine
but insufficient to react with the secondary amine groups to
any subst~ntial extent. The polyamide produced is then re-
acted with the epichlorohydrin to form the preferred poly-
amido-polyamine epichlorohydrin charge modifying agent.
~282~;65
rrypically, 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 polyamino-
ureylene containing tertiary amine ni-trogens 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 wlth epichlorohydrin (see for example the aforementioned
Munjat et al.)
The polyami.do-polyamine epichlorohydrin cationic resins
are available commercially as Polycup 172, 1884, 2002 or S2064
(Hercules; Cascamide Resln p~-420 ~Borden); or Nopcobond 35
(Nopco). Most preferably, the poly~mido-polyamine epichlorhydrin
resin is Polycup 1~84 or Hercules ~4308, whe.rein the charged
nitrogen atom orms part o~ a heterocyclic group:ing and is bonded
through methylene to a dependlny, reactive epoxide yroup. The
terms Polycup, Cascamide, Nopcobond and Hercules are all trade-
marks.
~ 25~5
21
Each monomer group in R 4308 has the general formula:
-_ CH _
--~ 2- CH CH
CH2 CH2
~,+ .
. ~ Cl-
_ ~ CH3CH2-CH-CH2
O __
Polycup 172, 2002 and 1884, orl the other hand, have
~onomer groups of the general formula:
r- R Cl- - !
. ~ !
- -- Ctc~2)4 --- CNHCH2 CH2 --~ NCH2 - CH2NH -
O O ' CH2
CH CH2
wherein R is methyl or hydrogen (Polycup 172 and 2002, R=H;
~nd Polycup 1884, R-CH3).
A secondary charge modifying agent may be used to
enhance the cationic char~e of the primary charge modifying
agent and/or enhance the bonding of the primary charge modi-
fying a~ent. The secondary charge modifying agent may be
selected from the group consistin~ of:
(i) aliphatic amines having at least one p.ri-
mary amino or at least two secondary amino groups;
and
,
56~
22
(ii) aliphatic amines having at least one
secoadary amine and a carboxyl or hydroxyl sub-
stituent.
Preferably, the secondary char~e modifyin~ agent is
a polyamide having the formula:
H
H2N-(Rl-N~ R2--NH2
wherein R1 and R2 are alkyl of 1 to 4 carbon atoms and x
is ar, inte~er from O to 4. Preferably, R1 and R2 are both
ethyl.
Preferred polyamines are:
Ethylene diamine H2N-(cH2)2-NH2-NH2
~iethylenetriamine H2N-(cH2)2-NH-(cH2)2-NH2
Triethylenetetramine H2N-(cH2-cH2-NH)2-cH2-cH2-NH2
Tetraethylenepentamine ~2N-(cH2-cH2-NH)3-cH2-cH2-NH2
The highly ~referred polyamine is tetraethylene pentamine.
Alternatively, aliphatic amines used in this inven-
tion may have at least one secondary amine and a carboxyl or
hydroxyl substituent. Exemplary of such aliphatic amines
are ~amlna-amino-butyric acid (H2NCH2CH2CH2COOH) and 2-amino
ethan~ 2Nc~2c~l2o~)-
The secondary charge modifyinK agent is bonded tothe micro~orous membrane by bondin~ to a portion of the epoxide
subs~ituents of the polymeric primary charge modifying agent.
The amount of primary and secondary cationic charge
,
, ~ modifying a~entsutilized is an amount sufficient to enhance
the electropositive capture potential of the microporous
membrane. Such an amount is hi~hly dependent on the speci-
fic char~e modifyin~ents utilized. For general guidance,
nowever, it has been iound that a wei~ht ratio of primary to
secondary charge modifyin~ a~ent of from about 2:1 to about
5~U:1, preferably from about 25:1 to about 75:1 is generally
sufficient.
In another embodiment of the present invention~ the
foregoin~ "secondary" char~e modifyin~ agent can be used as
'
,
565
23
tne char~e modifying agent by the cojoint employment of an
ali~hatic 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 1~6 to about 300. Such polyepoxides have viscosities
(undiluted) of less than about 200 centipoises at 25C. Due
to the necessity of the epoxide to act as a crosslinking
a~ent, 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 micropor-
ous membrane may be undesirable in the finished product.
Highly preferred polyepoxides have the formula:
R(O-C~2-CH~,CH2)n
wherein R is an alkyl of l to 6 carbon atoms and n is ~'rom 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 mix-
tur~s, e.~, u~ to 20% ethanol. While higher carbon content
materials are l'unctionally 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 cornposition which is capable of bonding to the membrane
microstructure without substantial pore size reduction or
pore blockage and provides an anionic charge or neKative
zeta potential to the membrane microstructure. Preferably,
such anionic char~e modifier is a water-soluble compound
havin~ substituents capable of binding to the membrane and
substituents which are capable of producin~ a more negative
"zeta potential" in the use environment (e.g. aqueous) or
anionic functional grou~s.
~Z82S6~
24
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 modi~ying 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.
H:ighly 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 mod:ifying agent may also be cross-
linked to the l~i.croporous membrane structure or itself in the same
manner as the catLonic agents using the sc~le allphatic polyepoxide
cross-linking agent having a molecular weight oE less than about
500. In additlon to the prefer:ced polyepoxides described above,
certain diglycidyl ethers of aliphatic diols,
CH -CH-CH -O-R-O-CH -CH-CH2
O O
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. m e terms RD-2,
Epi-Rez and Polyscience are trademarks.
~325~iS
~ ther hi~her carbon diglycidyl ethers may be used
as the polyepoxide cross-linking a~ent, for example 5-pen-
tallediol diglycidyl ether. However, the appropriate polar
or~anic solvents must be used for diluting such polyepoxides.
Tri~lycidyl ethers, i.e. tri-epoxides may also be
utilized as the polyepo~ide cross-linking agent. The tri-
e~oxides have the followin~ formula:
C~2~CH-C~12-0-CH2-CH-CH2-0-CH2-CH--CH2
C~2
Cll
. ,~ O
CH2
The tri~lycidyl ether of glycerol is available from Shell,
Inc. as Epon 812 and Celanese Corp. as Epi-Rez 5048.
Another preferred cross-linking agent is methylated
urea formaldellyde resin, commercially available from American
Cyan~nid; ~or example, Beetle 65, and melamine formaldehyde,
e.g " Cymel 303 ~rom American Cyanamid.
Other water~soluble polymers havin~ polar groups
can also be em~loy0d in this invention as the charge modi-
fyin~ a~ent. Examples include sodium alginate, ethylene
diamine tetraacetic acid, diethylene triamine tetraacetic
acid, tetraethylene pentamine tetraacetic acid, quaternized
polyethyleneimine, quaternized vinyl pyridine, quaternized
dIethylaminoethylmethacrylate and the like. The molecular
weight of the charge modifying agent does not appear to be
sl~nificant so lon~ as the a~ent is soluble in the polymer
"do~e". Thus, sodium alginate which has a molecular weight
above 10,000 and ethylene diamine letra acetic acid which
has a molecular wei~ht below 10,000 are equally employable.
~rne polyamido-polyamine epichlorohydrin cationic resins
generally have a molecular weight above 10,000. For example,
Polycup 1884 has a molecular weight of about 300,000 and
~4308 has a molecular weight of about 530,000.
`" ~2~32~i6S
26
Other surface rnodifying agents which are soluble or
suspendable in aclueous solvents are such materials as carbon,
diatomaceous earth, barium ferrite, iodine, aluminum, alumina,
silica, kaolin, molecular sieves, carbohydrates, perlite, clays,
vermiculite, asbestos, bentonite, casein and the like.
Broadly, the process of this inven-tion is directed to
surface modifying a hydrophilic organic polymeric microporous
membrane, e.g. nylon. The process comprises forming a dope
solution of nylon polymer, water-soluble or water-suspendable
membrane surface modifying agent and a solvent, diluting the
resulting dope solution with a miscible non-solvent for -the nylon
polymer and contacting the diluted dope solution with sufficient
non-solven'c for the nylon polymer to precipitate said membrane
therefrom. The dilution of the dope solution is preferably carried
out up to the point of incipient precipitation of the nylon but
should any precip:itation occur, the solids can be eliminated by
filtration or can be redissolved by addin~ additional solvent to
the d:iluted dope solution. When cast films a.re prepared, the
dilu-ted dope solution is spread on a substrate surface p.rior to
contact with the non-solvent :eor p.recipitation. When ~ibers c~re
being prepared, the contact:ing step is conducted by extruding the
clope :into a quench:ing bath ancl/o:r with the application of shear.
.~n orcle.r to provi.de the sur:Eace modifying amount of
surface modifying agent to the membrane, it is preferred that the
polymer dope solution contain at least about 0.01~ surface
modifying agent, by weight of total solids. The maximum amount of
surface modifying agent in the solution is limited by economic and
solubility-suspenclability limitations~ For example, an excess of
modifying agent which does not become bonded co the microporous
membrane will not be economically utilized and will constitute an
undesirable extrac'cive from the membrane. It has been found that
the amount of surface modifying agent in the dope should not exceed
about 75~ by weight of total solids.
~2~32~i6~
27
Af-ter the microporous membrane has been prepared, it is
then dried and cured, preferably in a restrained condition to
prevent shrinkage.
Drying of the membrane under restraint is described in
the Assiynee's defensive publication T 103,602 -to Repetti,
published November 1, 1~83. Generally, any suitable restra:ining
technique may be used while drying/ such as winding -the membrane
tightly abou-t a drying surface, e.g. a drum. Biaxial control is
preferred and tensioniny the membrane on a stretching frame is
considered the most preferred. Preferably, the restralning imposed
effects no reduction in dimensions.
Final drying and curing tempera-tures should be to dry and
cure the trea-ted membranes, preferably from about 120C -to 140C
for minimiza-tion of drying times without embrittlement or other
de-trimental effects to the membrane.
The completed membrane may be rolled and stored for use
under ambient condi-tions. It will be understood tha-t the treated
rnembrane may be supplied in any of the usual commercial fomls, for
example, as discs or plea-ted cartridges.
The presen-t invention provides an integral, coherent
microporous me~brane of retained internal pore geometry. The
surface modified membrane has an improved effective filtration
ra-ting relative to the untreated rnicro~reticulate polymer
structure.
For so-called sterile filtrations involving biological
liquids, the filter is sanitized or sterilized by au-toclaving or
hot water flushing. Accordingly, the surface modified membrane
must be resistant to this type treatment, and must re-tain its
integrity in use. Any modification to -the filter structure,
especialiy brought about by chemical agents which may be unstable
under conditions of treatment and use, must be scrutinized wi-th
care to minimize the prospect of extractables contaminating the
filtrate, interfering
. ` i~2~32S6~;
28
with analyses and potentially introducing harmful toxins to a
patient. Specifically, any such filter must meet the test
standards in the industry, e.g. ASTM D 3861-79, and generally prove
less than 5 mg. of extractables in 250 ml solvent (water at 80& .;
35% ethanol at room te~nperature) for a 293 mm diameter disc.
Biological liquids as that term is employed in the
specification and claims, is a liquid system which is derived from
or amenable to use with living organisms. Such liquids are
ordinarily handled and processed under sani-tary or sterile
conditions and therefore require sanitized or sterilized media for
filtration. Included within such term are isotonic solutions for
intermuscular (im) or intravenous (iv) administration, solutions
designed for administration per os, as well as solutions for
topical use, biological wastes or other biological fluids which may
comprise filterc~ble bodies such as impurities, e.g., bacterial,
viruses or pyrogens which are desirably isolated or separated for
examination or disposal by immobilization or Eixa-tion upon or
entrapment within filter media.
Filter membranes in accordance with this invention may be
employed alone or in combination with other filter media to treat
pharmaceuticals such as antibiotics, saline solutions, dextrose
solutions, vacc:ines, bloocl plasma, serums, (e.g. to remove hormones
or toxins), sterile water or e~e washes; beverages, such as
cordials, gin, vodka, beer, scotch, whiskey, sweet and dry wines,
champagne or brandy; cosmetics such as mouthwash, perfume, sharnpoo,
hair tonic, face cream or shaving lotion; food products such as
vinegar, vegetable oils; chemical such as antiseptics, insecti-
cides, photographic solutions, electroplating solutions, cleaning
compounds, solvent purification and lubrication oils, cutting oils
for removal of metallic fines (e.g. where the ferrite modifying
agent has been magnetized); and the like for retention of
submicronic particles, removal of bacterial contaminants and
: , ,
~2~325~5
29
resolution of colloidal hazes. Illustratively, in hospital
usage, membrane filters are employed to concentrate abnormal
exfoliated cells from a vaginal rinse, to isolate blood
~arasites from peripheral blood, or bacteria from serum or
leucocytes and casts from urine.
In the case of ~reparation for use in sterile fil-
tration, the membrane is thermally sterilized or sanitized
as by treatment in an autoclave at 121C. under 15 psig. for
1 hour, or hot water flushing at 85F. for 1 hour.
The membranes and fibers, etc. of this invention
can also be used to provide a bactericide (e.g. where the
modifying agent is iodine) or bacteriostatic treatment to
fluids, to remove contaminants such as chlorine or phenol
from fluids, in molecular separation columns, in bioreactors
where cells, etc. are immobilized thereon, as cigarette
filters, and for many other uses.
Havin~ now generally described khis invention, the
same will become better understood by reference to certain
s~ecific eamples, which are lncluded herein i'or the purposes
of illustration only and are not intended to be limiting oi'
t~le invention.
E~AMPLES
Tbe followin~ are the measurement and test procedures
utilized in all the Examples.
Thickness
.
The dry membrane thickness was measured with a 1/2
inch (1.27 cm) diameter platen dial indicator thickness gauge.
Gau~e accuracy was +0.00005 inches (+.05 mils).
Initial Bubble Point (IBP) and
Foam-All-Over-Point (FAOP) Tests
A 47 mm diameter disc of the membrane sample is
placed in a special test holder which seals the edge of the
disc. Above the membrane and directly in contact with its
up~er face, is a perforated stainless steel support screen
which prevents the membrane from deforming or rupturing when
~28~565
air ~ressure i-s a~plied to its bottom face. Above the mem-
brane and support screen, the holder provides an inch
deep cavity into which distilled water is introduced.
~ re~ulated air pressure is increased until a first stream
of air bubbles is emitted by the water wetted membrane into
quiescent pool of water. The air pressure at which this
first stream of air bubbles is emitted is called the Initial
bub~le Point (IBP) of the larJest pore in that membrane
sample - see AST.~ D-2499-66T.
Once the Initial Bubble Point pressure has been
determined and recorded, the air pressure is further in-
creased until the air flow throu~h the wetted membrane sam-
ple, as measured by a flow meter in the line between the
regulator and the sample holder, reaches 100 cc/min. The
air pressure at this flow rate is called the Foam-All-Over-
Poin~ (FAOP), and is directly proportional to the mean pore
diameter of the sample membrane. In this series of tests,
these two parameters (IBP and FAOP) are used to determine if
any chan~e has occurred in the maximum or mean pore size of
the membrane sample as a result of the charge modi~ying
process utilized.
Flow Rate Test
A ~7 mm diameter disc of the membrane sample is
placed in A test housin~ which allows pressuriæed water to
flow through the membrane. Prefiltered water is passed
throu~h the membrane sample at a pressure differential of 5
psid. A graduate cylinder is used to measure the volume of
water passed by the membrane sample in a one minute period.
In this series of tests this parameter is used in conjunction
with the IBP and F~OP to determine if any reduction in pore
size or pore blocka~e has occurred as a result of the charge
modifying process utilized.
Dye Adsorution Test
A 47 mm diameter disc of the membrane sample is
placed in a test housin~ which allows pressurized water flow
~2S~i5
31
through the membrane. me challenge solution consists of distilled
water at a pH of 7.0, and Metanil Yellow dye (color index CI#13065:
CAS587-98-4) for cationically charged membranes and methylene blue
(color index CI#52015: CAS61-73-4) for anionically charged
membranes. The dye inlet concentration is adjusted to produce a 76
percent transmittance at a wavelength of 430 nm, as measured on a
Perkin-Elmer ~lodel 295 Spectrophotometer for cationic membranes or
34 percent at 660 nm as measured on a Bausch & Lomb Spectronic 710
Spectrophotometer for anionic membranes. By means of a peristaltic
pump the challenge solution is flowed through the membrane sample
at a flow rate of ~8 ml/min. me transmittance of the effluent is
measured by passing it through a constant flow cell in the
aforementioned spectrophotometer. m e effluent transmittance and
pressure drop across the membrane is measured and recorded as a
function of time. me test is terminated when the effluent
transmittance increases to 85 percent for cationic membranes or 45
percent for anionic membranes of the inlet transmittance. In this
series of tests, the length of tirne that it takes to reach the 85
or 45 percent, transmittance in the effluent is called the
"brealcthrough" time. Since the Metan:Ll ~ellow and methylene blue
are low molecular weight charged dyes lncapcible of be:ing mechan-
ically ren~ved (~llterecl) by the membrane, this brec~lcthrough time
is proporti.onal to the charge adsorptive capacity oE the membrane
sample. This test is therefore used to deterrnine the effectiveness
of the charge modification techn:ique.
~tr~ b:e~ n ~ 3~ 9,
Extractables were detennined by ASTM D-3861-79. The
quantity of water-soluble extractables present in the metnbrane
filters was determined by immersing the preweighed men~rane in
boiling reagent grade water for an extended time and then drying
and reweighing the membrane. A control membrane was employed to
eliminate weighing errors caused by balance changes or changing
moisture content of the membrane in the weighing procedures.
Weight changes of -the control
.
(
~Z~ $~
32
membrane were applied as a correction factor to the weight
chan~e of the test membrane filters.
EXAI~PLE I
A. Pre~aration of Microporous Membrane
A representative nylon 66 membrane of 0.22 micrometer
nominal ratin~, havin~ a nominal surface area of about 13 m2
an Initial Bubble Roint of about 47 psi, a Foam-All-Over-Point
of about 52 psi was prepared by the method of Marinaccio et al,
U.S. Patent 3,87~,738, utilizing a dope composition of 16 per-
i'~ cent by weight nylon 66 (Monsanto Vydyne 66B), 7.1~ methanol
a~d 76.9~ formic acid, a quench bath composition of 25Z metha-
nol, 75~ water by volume (re~enerated as required by the method
of Kni~ht et al, U.S. Patent 3,928,517) a casting speed of
24 inches/minute (61 cm/min), and a quench bath temperature
of 20C. The membrane was cast just under the surface of
the quench bath by appllcation to a casting drum rotating in
the bath (9 to 10 mils as cast wet, to obtain 4.5 to 5.5
mils dry) and allowed to separate from the drum about 90 of
arc from the point of application, the self-supporting mem-
brane forrnin~ a shallow catenary to takeup. A portion of
the uniform opaque film was dried (in restrained condition
to resist shrinka~e) in a forced air oven at 89-90C. for 30
minutes.
B. Preparation of Char~e Modified
1. Membrane samples (dried and undried) were dipped
in a bath of Hercules 1884 polyamido-polyamine epichlorohydrin
resin (g% solids by wei~ht), and allowed to attain adsorption
e~uilibrium. The treated membrane samples were washed to re-
rnove excess resin and dried in restrained condition on a drum
at a tem~erature of 110C. for a period of about 3 minutes.
The treated membrane samples were compared for flow
and bubble point characteristics as follows, and found to be
essentially identical for treated and untreated samples,
evidencinO retention of pore and surface geometry. The
results are set forth in Table I.
~2~32565
33
TABLE I
Control (No Undried Dried
Treatment) Membrane Membrane
Thickness (mils) 4.25 4.58 4.83
Initial Bubble Poi~t (psi)43.7 44.7 44.7
Foam-A11-Over-Point (psi) 55.0 54.0 54.7
Thickness Normalized Flow
Rate (cc. mil/min. cm2 psi) 7.1 7.2 7.0
BET, N2 adsorptiOn 13.12 - 13.58
Thus, in terms of the morphological and hydrodynamic
parameters that control mechanical sieving,the filtration charac-
teristics of the treated membrances were essentially identical
with the untreated nylon membrane.
2. Similar characterizations were conducted on an-
other membrane sam~le, similarly prepared, but treated with 2$
~ercules R4308 resin (a free radical polymerized resin based
U~OII diallyl nitro~en-colltaining materials, reacted with epi-
chlorohydrin) in a bath adJusted to pH 10.5, overcoated with
.1% tetraethylene pentamine, dried, cured, washed and redried.
rhe results are set forth in Table II.
TABLE II
Control
~ Dried Membrane
Wet 528 635
~ry 860 960
Elon~ation (C~c)
Wet 140 100
~rr 95 40
Surface area of the treated and untreated membranes
remained essentially unchan~ed; tensile stren~th increased
with treatment with some loss in elon~ation. The treated
~25~;5
34
sneet was more flexible; creasin~ of the untreated sheet
resulted in cracking and splittin~.
C. Filtration Tests
The Hercules 1884 treated membrane samples (Example
I.B.l.) were subjected to the filtraton tests indicated below:
P~ro~en_Removal
Purified E. coli endotoxin was added to a 0.9~ NaCl
solution, pH 6.7 and passed through test ~ilters mounted in
a 25 mm diameter stainless steel holder. Inlet and effluent
endotoxin levels were determined by standard L.A.L. analysis.
Results are set ~orth in Table III.
TABLE III
Inlet Endotoxin Effluerst Endotoxin Level (pg/ml)
Filter Level (pg/ml) 10 ml. 50 ml 100 ml
Dried, treated
.~lembrane 15000 1000 1000 1000
Control -
Untreated 15000 10000 10000 10000
(P~ is "pico~ram")
Yirus ~emova1
~ S-2 bacteriopha~e was added to Houston Texas (U.S~A.)
~a~ water to produce a concentration of 3.4 x 105 PFU/ml (PFU
is "Plaque Formin~ Unit"), and 10 ml was passed through each
of the test filters mounted in a 25 mm diameter stainless
steel holder. Effluents were analyzed for viral content by
standard ~echni~ues. Results are set forth in Table IV:
TABLE IV
Total Viral PFU Virus Removal
Filter in Filtrate Efficiency (%)
~ried, treated
Membrane 100 99.997
Control - untreated 250000 26.4
Monodisperse Latex Filtration
The test filters were challenged with a 10 NTU dis-
~erslon (NTU is "nephlometric turbidity units") of 0.109
565
micrometer monodisperse latex (~DL) particles at a flow rate
of 0.5 g~m/ft.2 (.002 lpm/cm2), pH 7.0, R=21000-ohm-cm.
Effluent turbidities (NTU) were monitored and filtration
efficiencies were calculated from equilibrium effluent tur-
bidities. Results are set forth in Table V.
TABLE V
~ilter MDL Removal Efficiency
Undried, treated 97.3%
Control-u~treated 10%
Dye~Removal Efficiency
The test filters were challenged with a solution of
blue food coloring dye (FD & C No. 1). The solution had a
li~ht transmittance of 62.5% at 628 nm. The light trans-
mittance of the effluent was monitored and removal efficien-
cies determined (based on distilled water light transmit-
tance - 100%). Results are set forth in Table VI.
TABLE VI
Throughput (litres) to ~O,o
Transmittance
Undried, treated 1.99
~ried, treated 1.76
Control-untreated o
EXAMPLE I I
The cationically char~ed microporous membrane of
Example I.B. 1. is prepared by repeating the procedure of
~xample I.A. and incorporating the Hercules 1884 resin into
the do~e composition.
EXAMPLE III
A nylon dope solution was prepared containin~ 10%
nylon, 85.3~ ~ormic acid and 4.7~ methanol. About 28% of
~ercules 1884 resin based on the wei~ht of the nylon was
introduced into the dope solution. The resulting dope solu-
tion was extruded through an orifice which was in near proxi-
~ ~3256~
36
mity to a recirculatin~ quench bath stream of about 25% v/va~ueous methanol. The recirculating stream produces a mo-
derate shear on the dope solution entering the bath, thereby
producing fine fibrils from the dope solution. The resulting
fibers were blended at a ratio of 1:1 with coho cellulose
fi~er and 4.3 ~rams of the resulting mixture was felted into
~ads. The electrokinetic status of the pad was determined
using streaming potential techniques (Knight and Ostreicher,
Measurin~ the Electrokinetic Properties of Charged Filter
.~edia, Filtration and Separation, January/February, 1981,
pp. 30-34). The pad had a slope Mv/Ft H2O of -6.8, an in-
tercept of -90.70 and an apparent zeta potential of +0,33.
E~AMPLE IV
A~proximately 1 litre of a mixture of methanol and
formic acid in a weight ratio of 0.04 was prepared and allowed
to equilibriate for 1 hour. Then to four separate flasks,
150 ml of the solution was added. Thereafter, Hercules resin
1884 (35% solids) were added in amounts oi 1, 5, 10 and 15
millilitres and allowed to equilibriate in a water bath at
40C. for one hour with agitation.
A sufficient quantity of nylon was added to bring
the wei~ht percenta~e o~ the nylon to 8% based on the weight
o~ the methanol and acid and the flasks were shaken in a
water bath at 40C. until the nylon dissolved. The composi-
tions o~ the resulting doped solutions were:
Percentage
Methanol 4.1 4.1 4.0 4.0
Formic Acid 87.8 87.2 86.4 85.7
~ylon 8 7.9 7.8 7.8
1884 0.2 0.9 1.8 2.5
Cationically modified microporous membranes are pro-
duced repeating the procedure of Example I. A.
.
12~3~565
37
EXAMPLE V
Four dope com~ositions containin~ 39 grams of Nylon
6~ and the fol10wing other ingredients were prepared:
Dope Formic_Acid Gr~ns Water Grams 4308 Resin Grams Pentamine Grams
1 231.36 ~.6~ 0 0
2 231.36 16.916 10.263 2.46
3 231.36 4.193 20.526 4.92
4 231.36 24.719 0 4.92
Dope 2 contains one equivalent wei~ht of 4308 Resin
and triethylenepentamine per weight nylon, formulation 3
contains two e~uivalent weights of both resin and pentamine
per weight nylon and dope 4 contains two equivalent weights
of the pentamine alone. The dopes were placed in a jar mill
roller bath at 20C. until full dissolution. Following the
procedure of EXAMPLE I. A., two membranes were cast from
each do~e just under the surface of a quench bath ~30% meth-
anol, 7~% water by volume) by application to a casting drum
rotatin~ in the bath usin~ an 8 mil blade to drum depth.
The membranes made from each dope were separated from the
casting drum and rinsed in two successive wash baths o~
distilled water. The membrane sheets were then doubled over
on top of themselves while wet and mounted in restrained
condition to resist shrinka~e and placed in a forced draft
oven at 80C. for one-half hour. The membranes were then
subjected to .~etanil Yellow dye absorption tests, the results
of which are shown in Figure 1. Thereafter, the membranes
were ~subjected to flow, IBP and FOAP tests and the following
results obtained:
-
~8~
38
SampLe Flow (Ml/Min) IBP (psi) FAOP (psi)
l 1 72 53.5 85
2 78 45.5 82.5
2 1 39 59 53.8
2 59.5 41.5 53
3 l 103 25 30
2 138 25 30
4 1 6 90~ 90+
2 11 85.5 90
EXAMPLE VI
To 253.6 ~rams of a Nylon 66 membrane dope for amernbrane of a 0.45 micron nominal ratin~ containing 40.576
~rams of Nylon 6~, methanol and formic acid (16~ solids) was
aaded 1.159 ærams of Hercules 1884 resin (35% solids) to
~ive 1% resin based on the nylon and the resulting mixture
was a~itated until a clear solution was obtained. Membranes
were pre~ared followin~ the procedure of Example I.A., using
the do~e without the 1884 resin and the dope with the resin.
The membranes were dried under restrained conditions for 30
minutes at 86C. and their pro~erties were measured using
test wa~er which had been prefiltered through a 0.2 micro-
meter nominal ratin~ membrane. The results ~re shown in the
following table:
Flow cc/Min.-
Membrane Thickness ~si=an2IBP (PSi) FAOP (PSi)
Do~e without resin 4.13 2.~ 41.3 47.5
~o~e with resin 4.4 2.44 38.4 45.3
The membrane prepared with the dope which did not
contain the cationic 1884 resin had an IBP/FAOP ratio of
0.8~ while the membrane prepared with the resin had a r~tio
of 0.848.
s~s
39
EXAMPLE VII
A membrane dope was prepared by combining 1805.5
parts of Nylon 66 with 9479 parts of a mixture of methanol
and formic acid to obtain a 16% solids nylon dope. The mix-
ture was heated with agi~ation at 30C. for about 4 hours.
A quantity of Polycup 1884 was added to the dope
in a quantity such that the concentration of the cationic
charge modifyin~ resin was about l~o based on the weight of
the nylon. Cast membranes were then prepared using the pro-
cedure described in Example I.A. A portion of the resulting
wet membrane was dried in restrained condition as a single
layer in an oven at 85C. for 15 minutes. The resulting
nominal 0.22 micrometer rated membrane had a thickness of
.1 mils. Another portion of the wet membrane was folded
back onto itself and dried under restrained conditions in
the 85C. oven for 60 minutes. The resulting membrane was
7.8 mils thick. Prior to drying, the wet membrane had a
thickness of about 6.1-6.~ mils. The nominal pore size of
the membrane was 0.3 micron.
~82565
, EXAMPLE VIII
Followin~ the procedure of Example III, pads were
~roduced usin~ other surface modifyin~ agents. The agent,
blend ratio, number of ~rams felted and electrokinetic status
o~ the pads are shown in the following table:
Fiber to Grams Slope Apparent
~ H0 Ratio Felted ~v/Ft H20 Intercept Zeta Pot.
Alon 0.53 1.632~9 0.69 - 1.60
Asbestosl 0.83 2.55.1 -32.90 - 0.25
Asbestos2 l.00 4.414.2 32.30 - 0.69
Asbestos3 1.00 3.625.1 -35.00 - 1.22
Asbestos4 0.97 2.917.4 -33.50 - 0.84
Casein 1.00 3.220.1 59.00 - 0.97
Silica 1.00 3.025.0 20.66 - 1.21
Cabosil 1.00 8.032.2 53.50 - 1.56
Se~hade~ (G-75)r 1.005.5 6.3 -67.24 - 0.30
Bentonite 1.00 5.446.3 64.30 - 2.25
Diatomaceous
Earth D.E. 215 1.005.0 27.9 26.66 - 1.35
Kaolin 1.00 6.062.4 -70.20 - 3.02
Na-Al~inate 1.00 5.518.5 -15.82 - 0.89
Alumin~n 1.00 7.8-181.8 - 5.49 + 8.82
Carbon 1.00 4.457.5 -10.14 - 2.78
Carbon/1884 Resin 1.005.1 0.3 32.00 - 0.01
D~-215/1884 Resin 1.005.6 7.6 -26.30 - 0.37
Alwninwn (1~) l.00 3.6- 6.2 -50.99 + 0.30
1~84/5A Molecular
Sie~e 0.67 2.0-29.2 -30.70 ~ 1.42
~arium Ferrite 1.00 8.653.2 -89.50 - 2.58
E~'rA 1.00 5.519.6 - 4.34 - 0.95
rodinQ (Tincture) l.004.3 33.1 50.03 - 1.60
5~5
41
1: Arizona - not acid washed
2: Canadian - not acid washed
3: ~rizona - acid washed
~: Canadian - acid washed
EXAMPLE IX
Followin~ the procedure of Example III, fibers were
~re~ared from a 60 ml dope solution containin~ 4.8g nylon
with and without lOg powdered activated carbon. The fibers
were exposed for 16 hours to 150 ml of distilled water which
had been chlorinated to 450 ppm chlorine. The chlorine con-
tent o~ the water was then determined to be 360 ppm for the
water treated with the non~carbon containing fibers and 0.4
ppm for the water treated with the carbon containing fibers.
EXAMPLE X
Anionically charged microporous membranes are prepared
by repeating the procedure of Example I.A. and incorporating
the foLlowing into the dope composition:
4% polystyrene sul~onic acid and 2.7% ethylene glycol
di~lycidal ether;
1.3% polyacrylic acid;
0.88'~ polyacrylic acid and 0.12% polyoxyethylene-
~olyoxypropylene ~lycol;
3.6% polyacrylic acid (mw 104,000) and 1.3% hexa-
methoxy methylrnelamine resin.
E~AMPLE XI
Into a polymer dope solution containing about 8%
nylon 66, was suspended activated carbon (67w% of total solids).
Tbe sus~ension was allowed to flow by gravity into a 75%/25%
by volume water/methanol non-solvent through a small orifice.
The resultin~ ~ibrils were harvested, washed and then tested
for chlorine and ~henol removal from water. In both cases,
56~ii
42
the cayacity o~ the fibrils was about 90-95~ of the particu-
late carbon per se and at equivalent mass transfer rates.
The fibrils did not manifest the same degree of problems
encountered with finely powdered carbon which has very poor
hydrodynamic characteristics, is difficult to retain and
tends to mi~rate.
EXAMPLE XII
Followin~ the proc0dure of Example I, unmodified and
post-formation modified microporous membranes were prepared.
The post-formation modified microporous membranes were made
by dippin~ one of the unmodified membranes into a 2 w% solu-
tion of ~ercules Polycup 172 resin (0.24~ solids). The same
do~e was modified by the addition of 7 w~ of the Polycup 172
resin (0.84ao solids) and duplicate microporous membranes
prepared. When removed from the quench bath, the membranes
~ere air dried and then dried in a forced air oven at 40C.
for 16 hours. The five membranes were analyzed for integrity
by determinin~ bubble point~ F`OAP and then challenged with
Metanil Yellow dye. The results are shown in the following
table:
Bubble PSI Dye Ret. Time
~lombranePoint FOAP Initial Final (min.)
__
Unmodified40 46 3.0 3.6 7
3~ 44 1.9 ~.9 7
~lodified -
Post Treatment 44 50 3.9 5.0 24
~lodified Dope 31 52 3.9 12.0 77
31 53 4.3 14.0 73
Various changes and modifications can be made in
the process of the present invention without departing from
~le s~irit and scope thereof. The various embodiments which
have been described herein were for the purpose of further
illustrating the invention but were not intended to limit it.
