Note: Descriptions are shown in the official language in which they were submitted.
s~
SURFACE ~IODIFIED POLYAMIDE MEMBRAN~
The pres~nt invention relates to microporous membranes,
5 their preparation and their use. Microporous membranes
have been recognized for some time as useful for filtering
fine particles from gas and liquid media. United States
Patent Speci~ication No. 4,340,479, discloses a process
for manufacturing microporous polyamide membranes with cert-
10 ain desirable filtration characteristics. ~lembranes prepar-
ed by this process are hydrophilic, have narrow pore size
distributions and pore ratings as fine as about 0.04
micrometer. For many filtering requirements those membranes
perform very effectively. For certain fine particulates,
15 e.~.,substantially below 0.1 micrometer in diameter,
they are not effective. The reasons for this are rela~ed
to the mechanisms by which filters work.
The function of a filter is the removal of suspended parti-
20 culate material and the passage of the clarified fluid
medium. A ilter membrane can achieve fluid clarification
by different mechanisms. Particulate material can be rem-
oved through mechanical sieving wherein all particles
larger than the pore diameter of the filter membrane are
25 removed from the fluid. With this mechanism, filtration
efficiency is controlled by the relative size of the cont-
aminant and filter pore diameter and ~he efficient removal
of very small particles, e.g., less than 0.1 micrometer,
~ in diameter, therefore requires filter membranes with very
30 ~mall pore sizes.
-- 2
Such fine pore filter membranes tend to have the undesir-
able characteristics of high pressure drop across the
filter membrane, reduced dirt capacity and shortened
filter life.
A filter may also remove suspended part-culate n.aterial
by absorption onto the filter membrane surfaces. Removal
of par.iculate material by this mechanism is controlled
by the surface characteristics of (1) the suspended
particulate material and (2) the filter membrane. Most
suspended solids which are commonly subjected to removal
by filtration are negatively charged in aqueous s~stems.
This feature has ].ong been recognized in water treatme~nt
processes where cationic flocculating agents, Gpposltely
charged to the suspended matter, are employed to improve
settling efficiencies during ~ater clarification.
Colloid stability theor~ can be used to predict the
interactions of electrostatically charged particles ànd
surfaces. If the charges of suspended particle and the
filter membrane surface are of like sign and witll zeta
potentials of greater than about 2~nV~, mutual. repulsive
forces will be suf~iciently strong to prevent capture by
a~sorption. If the zeta potentials of the suspended
particle and the filter mem~rane surface are small, or
more desirably of opposite sign, particles will tend to
adhere to the filter membrane surfaces with high
capture efficiencies. Most suspensions of particles
~.,;Y
. ,,
''lj,
~L2~ S~
encoun~ered in industrial practice llave a nega~ive zeta
potential. Thus microporous filter membranes characterized
by positive zeta potentials are capable in a large number
of industrial applications of removing particles much
5 smaller than the pore diameters of the membrane through
the mechanism of electrostatic capture.
The desirable hydrophilic properties of the polyamide
membranes of U S. patent specification 4 340 479 are
10 believed to result in part from the high concelltration on
the exposed membrane surfaces of amine and carboxylic acid
end groups of ~he polyamide. The positioning of these gro-
ups is also believed to provide these membranes with their
unusual zeta potential versus pll profile. Tha~ ofile
15 positive at pHs below 6.5 becomes regati~e in alkallne
media. Accordingl~ these nlembranes ha e limited ability
to filter ver~ fine negatively charged partic~lates in
alkaline media.
20 By modif~ing the surface characteristics of the h~drophilic
membranes disclosed in U. S. patent specificati`on 4 340 479to
for instance provide a strongly positive zeta potential
over the alkaline range the spectrum of uses for these
materials in filtration is substantially exparcled.
The present invention is directed then to ~he preparation
and use of surface modified microporous h)~drophilic
pol~amide membranes. The process of this invention provides
s~
-- 4
m~croporous membranes with narrow pore size distributions
and fine pore ratings ranging :Erom 0.04 to
10 micrometers, preferably 0.1 to 5 micrometers, and
filtration efficiencies ranging ~rom molecular dimensions
(pyrogens) to particulates larger than the pore diameters.
The surface modified membranes of this invention with
their strongly positive zeta potentials are use~ul for
their greatly enhanced filtration efficiency over a broad
pH range and with a wide variety of contaminants including
]o particulates, particularly very ~ine negatively charged
particles, bacteria and endotoxins. ~;embranes o~ the
present invention also are capable o~ delivering ultrapul^e
effluent water rapidly after the onset of filtration. T~le
ability to deliver such high purity effluent water, .~ree
15 from microparticulate and ionic contaminants, makes the
products of this invention particularly desirable for the
filtration of aqueous fluids employed in microelectronics
manu~acture.
20 According to the present invention there is provided.a
process for preparing a polyamide membrane that is readily
wetted by water wllich process comprises the
steps of:
(1) preparing a casting solution including (A) a
25 casting resin system comprised of (a) an alcohol-insoluble
polyamide resin h~ving a ratio (CH2 :NHC0) of methylene
- C~l2 to amide NHC0 groups within the range of Erom 5:1 to
- 7:1, and (b) a cationic , water-solubi.e, quaternary
ammonium, thermosetting, membrane-surface-modifying
~L20~
polymer and (B) a solvent system in WhiCIl said castin~ resin
system is soluble;
(2) inducing nucleation of said castin~ solution
by controlled addition of a non-solvent for said casting
5 resin system under controlled conditions of concentration,
temperature, addition rate and degree of agitation to
obtain a visible precipitate of casting resin system
particles, thereby forming a casting composition;
(3) spreading said casting composition on a subs-
10 tr~te to form a film thereof on the substrate;
(4) contacting and diluting the film oE saidcasting composition ~ith a liquid non-solvent system for
said casting resin system comprised of a mixture o~ solvent
alld non-solvent liquids, thereby precipltating on to the
15 s-lbstrate said casting resin system from said casting
composition in the form of a thin, skinless, hydrophilic,
surface-modified, microporous, polyamide membrane;
(5) ~ashing said membrane to remove the solvent;
and
(6) drying said membrane.
The surface modi~ied, alcohol-insoluble polyamide nlembralles
in accordance with this invention have the unusual property
of being hydrophilic, i.e., they are readily wetted by
25 water, have pore sizes (also referred to as pore ratings
or pore diameters) of from 0.04 to 10 (prefer-
ably 0.1 to 5 micrometers) or more, modified zeta
potentials, i.e., strongly positive zeta potentials in
,.,~
5~
-- 6
all~aline media~ filtration efficiencies ranging from
molecular dimensions (pyro~ens) to particula~es larger
than the pore diameters and, accordingly, are highly
desirable as fil~er media, particularly for producing
bacterially sterile filtrates, as well as for filtration
of high purity water in microelectronics manuacture due
to their ability to deliver ultrapure filtrate free from
microparticulates and ionic contaminants.
T7-e membrane surface modifying polymers or resins useful
in preparing the membranes in accordance with ~l~is
invention are the cationic, water-soluble, q~la~elnar~
an~nonium, thermosetting polymers, Preerred pol!mers
within this class are the epoxy-functional pol~amlde/
polyamido-epichlorohydrin resins The epox!-lut~ctional
polyanine epichlorol-ydrin resins are partic~larly preferr-
ed.
The sole figure is a graph of percent added surface
modifying polymer versus time required to ob~ain 14
megaohms/cm effluent water from a surface modified
membrane in accordance ~7ith this invention.
The subject invention is directed to surface modified,
hydrophilic, microporous, polyamide membranes and a
process for preparing them by th~ steps of (1) preparing
~ a casting solution comprised of (A) a resin casting system
- comprised of ~a) an alcohol-insoluble polyamide resin
having a ratio of CH2 :NHC0 of metllylene CH2 to amide
~CO groups within the range of from 5:1 to
.
7:1 and (b) a membrane s~rface modifying polymer;
and (B) a solven~ system in which the casting resin
system is soluble; (2) inducing nucleation of ~he
casting solution by controlled addition of a non-solvent
for the casting resin system under controlled conditions
of concentration temperature addition rate and degree
.of agi~ation to obtain a visible precipitate of casting
resin s~stem particles which may or may not thereafter
partially or completely redissolve thereby Iorming a
casting composition; (3) preferabl~ filterirg tle casting
c~mposition to remove visible precipitated particles;
(4) spreading the casting composition on a subs~rate to
for.n a thin film tllereof on the subs~rate; (5) contactin~
and diluting the film of casting composition with a liquid
non-solvent system conprised of a mixture of solvent
and non-solvent liquid and containing 2 substantial prop-
ortion of the solvent liquid but less than the proportion
in the casting solution thereby precipitating tile casting
resin system from the casting solution in the form of a
thin skinle$s hydrophilic surace modifiecl microporous
membrane; (6) washing the membrane to remove solvent; and
(7) drying the membrane
The polyam.de ~em~ranes of U. S. Fatent Specificatlcn 4 340 479
are prepared from alcohol-insoluble polyamide resins
having a n-.ethylene to amide ratio in the range of
5:1 to : . 7:1 as are the surface modified membranes
in accordance with this invention~ Membranes of this
group include copolymers of hexa-methylene diamine and
adipic acid (nylon 66) copolymers of hexamethylene
. . ..... ~ . ... ..... , , , . , , _
~Z~ 5~3
diamine and sebacic acid (nylon G10) homopolymers of
poly-e-caprolactam (nylon 6) and copolymers of he~amethylene
diamine and a~elaic acid (nylon 69). Nylon 66 is preferred.
5 In the process for manufacturing the membranes of U. S.
Patent specification 4,340,479, the polyamide resin is
dissolved in a solvent, such as formic acid, and a non-solv-
ent, such as water, is added under controlled conditions of
agitation to achieve nucleation of the solution.
In inducing nucleation of the polyamide solution a visible
precipitate is formed. This precipitate may partially or
completely redissolve. Preferably, any visi.b].e particles
which do not redissolve should be filtered out o the sys-
tem, e.g., with a 10 micrometer filter, prior to casting
the nucleated solution or casting composition.
The nucleated solution or casting com~osition is tllen cast
onto a substrate, e.g., a porous polyester sheet of web
20 or a non-porous polyester sheet, in the form of a film
and this film solution is then contacted with and diluted
by a liquid non-solvent s,~stem which is a rllixture of a
solvenL and a non-solvent for the polyamlide resin. A
preferred non-solvent liquid system for both the subject
25 in~ention and that o.~ U.S. Patent specification L~,3~0,L~79
is a solution of water and fonnic acid. For this invention,
..:
'.
'~
~g'3g~S~
the formic acid is preferabl~ present in an amount of
from about 35% to about 60% by weight~ The polyamide
resin thereupon precipitates from the solution, formin~
a hydrophilic membrane sheet on the substrate which can
5 be washed to remove the solvent. The membrane can then
be s~ripped from the substrate and dried or, iE the
substrate is porous, it can be incorporated in the membrane
~o serve as a permanent support, in which event it is
dried with the membrane. I the subsTtrate is to be in-
10 corporated into the membrane, it should be porous andcapable of being wetted and im~re~nated by the casting
composition, e.~., a porous, fibrous, ~olyester sheet
with an open structure. By appropriate control oE
process variables, membranes with thro~h poles of uniorm
15 si~e aTld sllape can be obtained. Con~ersel~y, i.f (~esired,
tapered through pores, wider at one surace of the sheet
and narrowing as they proceed toward the opposite surface
of the sheet, can be obtained.
20 The same ~eneral proced-~re described above is follo~ed
in manufacturin~ the surface modified Tnembranes in
accordance ~ith this invention except that tlle membrane
surace modif~in~ pol~rmers used in thc subject invention
are combined with the polyamide resin,and the resulting
25combined modifying polymer/polyamide casting solution,
after nucleation to form the casting composition, is cocast
resultin~ in unique membranes with novel filtration
properties extending the range of uses for microporous
polyamide T~ranes.
.
~'
lZa~S~
,,
- 10
The novel properties of the filter membranes prepared by
the process of U. S. Patent specification 4,340,479 are
5 believed to result in part from the high concentration on
the membrane surfacesof amine and carboxylic acid
end groups of the polyamide. These amine and carboxylic
acid functions on the membrane surfaces result in unexpect-
ed membrane properties, such as their unusual zeta potent-
ial versus pH profile and their hydrophilic character, thatis beingreadily wetted by water, typically being wetted
throu~h in 3 seconds or less, preferably 1 second or less,
~hen i~-nersed in water.
As previously stated, it has now been discovered that the
surface modified membranes in accordance with this invention
having unexpected and novel filtration properties can be
prepared using the general procedure disclosed in U. S.
Patent specification 4,340,479 but with the addition of
low levels of se]ected membrane surface modifying polymers
to the polyamide membrane casting solutions. ~lus, surfac-
e-modified, hydrophilic, microporous polyamide membranes
with a stron`gly positive zeta potential in allialine media,
having low levels of extractable matter, and havin~ the
ability to deliver ultrapure water, free from micropart-
iculate and ionic contaminants, quickly after the onset of
filtration as required in microelectronics manufacture,
are readily and economically prepared by the cocasting
process in accordance with the ~resent invention.
Addition of as little as one weight percent, based on
the polyan,ide resin, of the membrane-surface-n)odifying
pol~er to the membrane casting solution has been found
to producemicroporous hydrophilic membranes wl-ose surface
5 properties are substantially controlled by the modifyin~
polymer. It is the ability of relatively small amounts
of the membrane surface modifying polymer to control the
surface pro~erties of membranes in accordance with the
present invention which provides the desir~ble character-
10 istics of the sub~ect membranes. Accordingly, thefiltra~ior, cha-acteris ic and th_ physiochemical surface
behaviour of these membranes are controlled b~ a surprisin~-
ly low proi-ortion of the modifying polymer.
15 The (1~ sul-face modified, pol~amide membranes prepared
using the cocasting process share certain characteristics
with both (2) the base polyamidc membranes prepared using
the same general casting process but without the modifying
pol~mer present in the casting solution and (3) base
20 polvamide membranes prepared in the SaTne m~cnner as the
membranes of (2) above but which have subsesuently been
coated ~ith a cationic polymer or resin by contacting the
finished base polyamide membrane with a solutioII of the
cationic polymer or resin.
~5
A11 three types are skinless, hydrophilic (as herei.nbefol-e
delil,ed all~ in U. S. Patent specification 4,340,479)
and microporous. Each has desirable filtering character-
istics for fine particulates derived in part from their
30 fine pore ratings and narrow pore size distributions. Un-
~ like the base polyamide membranes of (2) above, llowever,
. :~
~z`~ s~
- 12
che surace modi.fied polyamide membranes of (1) above and
the coated polyamide membranes of (3) above e~hibit a
positive zeta potential in alkaline media which broadens
the spectrum of uses of the membranes of (1) and (3).
Surface rnodified polyamide membranes in accordance with
this invention and base polyamide membranes (2) also
possess the ability to deliver ultrapure water, free from
microparticulate and ionic contarninants, quickly after
10 the onset of filtration. Conversely, the coated pol~amide
membranes (3) do not~have this ability to as great an
extent as either (1) or (2). This ability, as described
in detail hereinafter, is highl~ desirable in microelectr-
onics rr,anufacturing applications. Since only the sur.race
15 modified polyamide membranes (1) in accordance wiLh LhiS
invention have the unique combination of a posiLive ~eta
potential in allcaline media and the ability to del,ver
ultrapure water, free from microparticulate and ionic
con~aminants, quicl<ly upon the onset of filtration, the
desirability of this class of mernbrane is manirest.
The highly desirable properties of rr,embranes iII accord-
ance with the invention are believed to resul~ from the
unique method of preparation in which the mo~lif~ g poly-
mer becomes an interal part of the overall s~ructure of
the membrane. The abilit~ to prepare ~hese n~embranes
in a clean, straightforward, effi.cient and economic manner
heightens ~heir desirability.
The membrane-surface-modifying polymers or resins (some-
times herein referred to as "modifying polymer(s)") useful
~ 13
in processes in accordance with thls invention are the
cationic, water-soluble, quaternary ammonium, tllermosettillg
polymers. The preferred modifying polymers are those
polymers which undergo cross-linking reactions through
5 reaction of epoxide groups. Epoxide functional cationic
polymers or resins generally produce charge-modified sur-
faces which, upon proper conversion to the cross-linked
state by the application of heat, are found to be mechan-
ically strong and chemically resistant to a wide range
10 of aggressive chemical environments.
Epoxide functional or epoxide-based cationic thcrlnosetting
polymers are also preferred due to favourable interactions
believed to occur between the amine and carboxvlic acid
15 end groups of the polyamide. ~mine functions alld carbo~-
vlic acid functions are Icnown to co-react efficiently with
epoxide f-lnctional polymers. It is bèlieved that the amine
and carboxylic acid groups of the polyamide resin react
~ith the epoxide groups of the modifying polymer. While
20 it is believed that the reaction of these groups occurs
throu~hout the membrane structure, the nature of the memb-
rane forming process is believed to cause preferential
orientation of the modifyin~ polymer towarcls the surfaces
of the formed meulbrane. By this is meant tl)at as a
25 resu].t of the cocasting process in accordance with the
invention,the modifyirlg polymer determines tlle s~lrface
characteristics of the membrane. ~urther, the reaction of
the groups is believed to result in intimate bonding of
the modifying polymer and the polyamide resin forming
30 an integl~lstructure thereby reducing the amour-t of
extractable matter,increased homogeneity of the surfaces
and increased general stability of the membranes.
Another desired characteristic of the modifying poly~ers
relates to the nature of their cationic char~e. Since
,~
;
.z~3~-SB
14
their cationicity stems from the presence of guaternary
a~monium groups, t11ey maintain their positive charge in
acid, neutral and al1~aline conditions of pH.
Surprisingly, during t11e membrane formation process, the
lo~.~ added levels of modifying polymers appear to be
preferentially orientated in such a manner as to result
in membranes whose surface characteristics are substantia-
lly controlled by the 7nodifying polymer~ This result is
believed to re~lect both the membrane forming process and
the hydrophilic nature of the modifying polymers. The
combinatior. of their llydrophilicity, their apparellt stron~
interaction ~ith t11e polyamide end groups and the cocast-
ing process are believed responsible f~r ~he a~parerlt
preferential orientation of tlle modifyillg polylrers to-
wards the membrane surface.
Also, surprisingly, while the modifyin~ ~olymers are llighly
soluble in water, the~ are not leached out of t11e casting
composition into the non-solvent liquid, wnich is used
to precipitate the casting resin system. Apparent:lY,
the scrong inter-action of the modifying pol~rmer with the
polva~ide end groups coupled with the preferential orient-
ation of the modifying ~olymer towards the membrane
surfaces (both pore and external), perhaps under the in-
fluence of the non-solvent liquid, combine to provide a
membrane whose surface properties are substantially contr-
olled by the cationic, quaternary ammonium groups of the
modifying polymer. The unexpected result is hig111y desir-
able.
- 15
The epoxy-functional or epoxy-based resins preferred fall
into two classes: polyamido/polyamino-epichlorohydrin
resins and polyamine-epichlorohydrin resins. The former
are reaction products of epichlorohydrin with polyamides
containing primary, secondary and tertiary amines in
the backbone. Representative materials of ~I-is class
are described in United States Patent Specifications
2,926,154, 3,332,901, 3,224,986, and 3,855,15S.
Preferred commercially available water-soluble, non-
colloidal cationic thermosetting polymers of the polya-
midolpolyamino-epichlorohydrin class, are ~Tmene 557
(Registered Trade Mark) and the Polycup ~Regis~el^ed Trade
~iark~ series of resins manu~actured by ~lercules lncorpor-
ated. I~ is believed that these resins are preral^ed byreacting epichlorohydrin with 10~7 molecular weig~lt
polyamides which contain amino groups in the polymer
backbone. The reaction products have been described
as containing quaternary ammonium groups present in
the form of aze~idinium ions which are four-membered
ring structures. Kymene 557 and the mernbers o~ ~he
Polycup series have been described as being chemically
and structurally similar but differing in ~heir molecular
weight.
Polyamine-epichlorohydrin resins are condensation
products of polyamines such as polyalkylene polyamines
. "
..... --_ .. ...... .
.. . . .. . ...
~z~ s~
- 16
or their precursors with epichloroh~drin. They differ
from the polyamido/polyamino-epichlorohy~ ~ resins in
that the polyamine-epichlorohydrins contain no amide
linkages as part of tlle backbone of the polymer. Commer-
5 cially available compositions of this type are dîsclosedin (1) ~l.S. Patent Specification 3,885,158, ~hich dis-
closes polymers prepared by reacting epichlordlydrin
~ith the condensation product of pol~alkylene polyamines
and ethylene dichloride and in (2) U.S. Patent Specificat-
10 ion 3,700,623 which discloses polymers made byreacting epichloro~,drin with polydiallymethylamine.
Compositions of the irst type are exemplified b~ Santo-
res 31 (Registered Trade ~lark) (Monsallto, Xnc) and
compositions of the second type are exempli~ied hy
15 Resin R4308 (Hercules Inc.):
Especially preferred are the epoxide functional water
soluble cationic polymers which fall in the class of
polyamine-epichlorohydrin resins and which bear
20 quaternary groups in the cured state. The fact tllat
quaternary groups remain in the resin in the cross-linked
state is important, since it affects the pH range over
which the membrane can maintain a positive zeta potential.
A quaternary ammonium group is inherently cationic;
25 hence its positive charge is independent of its pH
environment. Resin R4308 and Santo-res 31 each bear
quaternary ammonium groups in the cured state and bear
a positive charge at alkaline pH.
~z~s~
- 17
~lany of the modifying polymers require activation For
the purpose of providing extended shelf life or storage
stability to these resins, the epoxide groups are
chemically-inactivated to prevent premature cross-link-
ing of these polymers. Thus, prior to the use of these
pol~rmers the polymers are activated into the reactive,
thermo-setting state by regeneration of the epo~ide
groups. Typically, activation entails adding sufficient
aqueous caustic to a solution of the inactive polymer to
chemical~y convert the inactive chlorohydrin ~orm to the
cross-linking epoxide form. The parts by ~i7eigllt of
aqueous caustic per weight polymer var~ with the product
and are specified by the manufacturer. Complete
activation is generall7~r achieved in about thirty mir-utes.
The preparation of membranes in accordance with this
invention is carried out under controlled conditions
including addition o~ the non-solvent, e.g., ~7ater, to
a solution of the polyamide and the modifying polymer,
control of the concentration of the constituents,
control of the temperature and control oE the agitation
of the system to induce the proper level of nucleation.
The detailed discussion in U.S. Patent speci~ication
~1O. 4,340, 479 concerning the relationship of the
parameters set out above is gen~-ally applicable herein
and will not be repeated
';
~20~51~
Tlle manner and rate of addition of the non-solvent to
induce nu~leation is interrelated with other process
variables, such as intensity oE mixing, temperature and
the concentration of the various components of the
casting solution ~ The term "casting solution" is ~ced here
to mean the solution made up of (A) the cast~ g resin
system and (B) the solvent system, Addition of the non-
solvent is conveniently carried out through an orifice
at a rate sufficient to produce a visible precipitate
which, preferably, at least in part subsequently redissol-
ves. Maintaining all other parameters collstcnt~,
casting compositions with quite diEferent cllaracteristics
in terms of pore sizes of the resulting membranes will
be o~tained b~ varying the diameter of ~he orlfice, The
required degree of nucleation resulting fron non-solvent
addition rate and orifice configuration is tllere~ore best
established by trial and error for each given system,
The controlled addition of non-solvent is discussed in
detail in ~l,S, Patent s?e~ifica~on ~o. 4, ~ 79.
Prior to addition of the non-solvent to induce nucleation,
tlle casting solution is prepared comprised o (~) a casting
resin system comprised o (a) an alcohol-insolu~le polya-
mide resin as hereinbefore described and (b~ a modifying
polymer or resin and (B) a solvent system, The solvent
system may simply be a solvent for the casting resin
system, e,g,, formic acid, Alternatively, the solvent
system contains an amount of a non-solvent, e,g,, water.
The amount of non-solvent present in the casting solution
- ~.
5~3
- 19
is always less than the amount necessary to affect the
stability of the solution.
Prior to casting nucleation of the casting soluLion
5 is initiated by controlled agitation and the controlled
addition of non-solvent liquid. The amount and rate of
addition of non-solvent is controlled along with the
intensity of mixing of agitation. The ad~-antage of
inciuding non-solvent as part of the solvent system in
10 making up the casting solution is that better control of
the addition of non-solvent can be maintained during the
inducement of nucleation because sr.~.aller amounts of non-
sol~7er.t are needed due to the non-solvent alread~ present
in the casting solution. As a result better control of
the addition rate can be rnaintained ancl a more uniform
product of any desired pore size can be obtained.
The casting resin s~stem includes (a) an alcohol-insoluble
pol~amide resin having a methylene to amide ratio of
i-;orl 5:1 tG 7:1 and (b) a surface modifyillg polymer
or resin. All parts and percentages are by weigllt unless
otherwise stated.
The proportion of modifying polymer to polyamide resin
in the casting solution forr.led as the first step in the
process based on the polyamide resin~ can vary from as
much as 20 weight percent to as little as 0.1 weight percent
that is 20 parts of modifying p~l~er to ~00 pl.tS polya~.i de
`"!
.~
~z~
- 20
resin rangil-g to 0.1 part of modi~ying polymer to 100
parts polyamide resin. The generally preferred range of
added modifying polymer is from about 1 weight percent
to 5 weight percent. Addition levels of
5 1 to 2.0 weight percent have been found particularly
desirable. It is believed that this moderate le~el of
modifying polymer produces substantially complete membrane
surface modification resulting in a membrane ~hose surface
characteristics are substantially controlled by the cation-
10 ic, quaternary, am~onium groups of the modifying polymer.Thus, for the purpose of membrane efficiency and production
economy, the addition of about 1 to 2.0 weight percent of
the modifying pol~ner, based on the polyamide resill, is
preferred with the polyamide resin present in the castillg
15 solution in an amount of from 10% to 187c and tlle
surface modifying polymer present in an amount of
IlOm O.].% to 0.9%, (based on all components present
in the solution).
20 The amount of solvent present in the casting solution for-
med as the first step in the process will var~ dependent
upon the polyamide resin and the modifying polymer used.
In general, the amount of solvent present will rallge from
~iO to ~0 percent (based on all componen~s present
25 in the solution).
It should be understood that the casting solution comprises
both (1) the casting resin system , i.e., the polyamide
resin and the modifying polymer or resin and (2) the
30 solvent system, i.e., a solvent for the polyamide resin/
.
~z~
_ 21
modifying pol~ner casting resin system tsuch as formic
acid) and, if desired 9 a minor amount of a non-solvent
for the casting resin system (such as water),
The amount of non-solvent present in the casting solution
will in all cases be less than the amount in the liquid
non-solvent system (membrane forming bath) used
to precipitate the casting resin system from the casting
composition, the casting composition being the composition
formed from the in;tially prepared casting solution by
inducing nucleation in that solution and, preferably,
removing visible particles fro~ the resulting composition.
Gen-rally, when the non-solvent is water, it will be
present in tl~ casting solution in an amount ranging from
zero, preferably at least 5 percent, preferably 10 to
20 percent, and up to about 30 percerlt by wei~ht (a~ain
based on all the components present in the solution).
For a preferred casting solution, a polyamine epichloro-
hydrin resin or polymer, preferably Resin R4308, is pre-
sent in the casting solution in an amo-lnt o from
0.1 to about 0.9 percent, polyhexamethylene adipamide
is present in an amount of from 10 to 18
percent, formic acid is present in an amount of from
65 to 75 percent, and water is present in
an amount of 10 to 20 percent, ~11 parts by weight and
based on the total composition of the castin~ solution
'~LZ~3`~
22
The temperature of the casting composition is not
critical sc long as it is maintained at a constant
value. 6-enerally, ho~ever a decrea5e in casting
composition temperature produces a higher degree of
5 nucleation.
The intensity of mixing in a given system is a function
of a large number of interrelated variables. For any
~o given system, the mixing intensity can be expressed in
terms of the rotation rate of the agitator. Such
equipment has many forms and designs commonly used in the
mixing art and is difficult to quantify. Thus, trial
and error experimentation involving custom6ry variables
is necessary to establish the operable range oE mix;ng
intensities suitable for a particular s~stem. Typically,
using a 6.35cm rotor operating at ~ throughput of abut
500 to 1500 grams of solution per minute requires mixing
speeds in the range of from 1500 to 4000 RP~I to
20 produce membranes witll pore rating in ~he range oE
interest.
The liquid non-solvent system used to dilute the film
of casting composition and thereby precipitate the
25 casting resin system, typically ~y immersion in a bath
of the liquid non-solvent system, can, and preferably
~ does, contain a substan~ial amount of a solvent for the
casting resin system, preferably the one present in the
casting solution. That is, the liquid non-solvent
~0~15t~
_ 23
system is comprised of a mixture of a non-solvent for
the casting resin system, e.g., water, and 3 solvent
for the casting resin system, e.g., formic acid. However9
on a percentage basis the amount of solvent present in
the liquid non-solvent system will be less than the
amount present in the casting solution. Typically, the
liquid non-solvent system will be comprised of a non-
solvent, eOg., water, present in an amount ranging rom
about 65 to about 40 weight percent and a solvent for
tThe casting resin system, e.g., formic acid, present
in an amount ranging from about 35 to about 60 weight
percent. The formic acid present in the casting solution
may amount to from 60 to 80% and the water present
therein from O to 30% by weight. ~tore normally expressed
15 ~he formic acid range is from 65 to 7S% and the water
range from 10 to 20%. Preferably,:the bath of the liquid
non-solvent s~Tstem is maintained at a substantially con-
stant composition with respect to non-solvent and
solvent by the addition of non-sol~ent to the bath,
preferably continuously, in a quantity sufficient to
compensate for solvent diffusion into the bath from
the thin film of casting composition.
The solvent comprising at least part of the solvent s~s-
tem used in the casting solutions can be an~T solvent
for the casting resin system, .i.e., the combination of
the polyamide resin and the modifying polymer. A preferr-
ed solvent is formic acid. Other suitable solvents are
3~zll~L5~
_ 24
other liquid aliphatic acids, such as acetic acid and
propionic acid; phenols 9 such as phenol, the cresols
and their halogenated derivatives; inorganic acids,
such as hydrochloric, sulfuric and phosphoric; saturated
aqueous or alcohol solutions of alcohol-soluble salts~
such cs calcium chloride, magnesium chloride and
lithium chloride; and hydroxylic solvents, including
halogenated alcohols.
The only criteria in selecting a solvent are that (1)
it should form a solution of the polyamide resin and
the modifying polymer, (2) it should not react chemically
with either the polyamide resin or the surface modifying
polymer and (3) it should be capable of ready removal
from the surface modified polyamide membrane. Practical
considerations also are important, of course. ~or
e~ample, inorganic acids are more hazardous to work with
than are others of the named solvents and corrosion
problems must be dealt with. Since fO rmic acid meets
the criteria listed above and is a practical material
as well r it is the solvent of choice. Due to economy
and ease of handling, water is the non-solvent of
choice for use in the solvent system when a non-solvent
is used in the solvent system. In like manner, the
preferred non-solvent added to the castin~ soiution to
induce nucleation thereof is water~ And, the preferred
non-solvent component of the liquid non-solvent system
used to precipitate the casting resin system from the
film of casting composition is also water for the same
s~
- 25
reasons it is the non-solvent of choice in the solvent
syste~
The membrane products are characterized by being hydroph-
5 ilic, skinless,-microporous and alcohol-insoluble, with
narrow pore size distributions, filtration efficiencies
from molecular dimensions (pyrogens) up to particulates
larger than the pore diameters, pore size ratin~s of
from Ø04 to 10 micrometers, a preferred
10 ran~e being from 0.1 to 5 micrometers, film thicknesses
in the range of from 0.01 to 1.5 millimeters,
preferably from 0.025 to 0.8 mm, and by having
a positive zeta potential over a broad pH range of from
3 to 10. The pores may extend substantially
15 uniformly from surface to surface both in size and sha?e
or they may be wider at one surface than the other and
thus taper bet~een the surfaces. Additionally, the membr-
anes can be characterized as having ~urface properties
which are substantially controlled by cationic quaternar~
20 ammonium ~roups of the cationic, quaternary a~monium,
thermoset, surface modifying polymer. Surprisingly,
the low added levels of quaternary an~onium, surface
modifying pol~ers produce membranes whose sur~ace
characteristics substantially reflect the presence of the
25 cationic quaternary all~onium groups. Along with their
excellent pore structure and positive zeta potential,
these membranes have ~ery low levels of extractable matter
making them especially desirable in pharmaceutical and
electronic filtration applications. Additionally, these
~ 30 membranes can be conveniently and economically
12~
- 26
prepared by a straightforward, continuous process as
described hereinafter.
The surface modified membranes with moderate or low levels
5 of modifying polymer have rinse up times to deliver eff-
luent water having a resistivity of 14 megaohms/crn of 10
minutes or less, and preferably less than 5 minutes,
when tested using the Resistivity Test described hereina-
fter.
Surface modified membranes particularly those ;ith
moderate or low levels of surface modifying pol~ners
present, are `believed to have quick rinse up tilTIes
because of the apparent interaction between the surace
15 r,odif,~ing pol~er and the polyamide resin end groups.
Tilis i.nteraction and the resulting integral nature of mem-
branes in accordance with the invention is belie~ed to
lead to a red~ction in the extractables available for
sloughing off and being carried through the filter and into
20 the effluent, a phenomenon belie~!ed to occur with coated
membranes.
~lethod of Testing tile Sur~ace ~odified
~lembranes of the Followin~ Examples:
The properties of the membranes of tlle following examples
were evaluated by a variety of test methods, as
described below:
(a) Zeta Potential.
The zeta potentials of membranes were calculated from
measurements of the streaming potential generated by flow
i' j
~2~
- 27
of a 0.001 weight percent solution o~ KCl in distilled
water through several layers of the membrane secured in
a filter sheet or membrane holder. Zeta poten~ial is a
measure of the net immobile electric charge on a membrane
surface exposed to a fluid. It is related to the
streamin~ potential generated when the fluid flows
through the membrane by the following formula (J T.
Da~is et al, Interfacial Phenomena, Academic Press,
~ew ~ork, 1963):
Zeta Potential (mV) = 4 ~ . Es
D P
Where ~is the viscosity of the flowing solution, D
is its dielectric contsant, A is its conductivit~, Es
is the streaming potential and P is the pressure drop
across the membranes during the period o ~low. In the
~ollo~ing examples, the quantity 4~?r~ was constant,
having a value 2.052 x 10-2 , oDr when converted
to Kg /Sq. M., the constant must be multiplied b~ the
conversion factor 703.1 so tha~ the zeta potential can
b~ expressed:
Zeta Potential (mV)=
14.43 . E~ (Volt) .~ nho/cm)
P (Kg/cm2 )
(b) Capacity of Membrane for Latex Adsorption:
A 47mm diameter disc of ~er~rane was placed in a
filter holder with a filtration area of 9 29cm2 and
~ was then challenged with a suspension of 0.01 weight
percent monodisperse latex spheres in a 0.1 weight
percent Triton X-100 in water solution (Triton X-100 is
, l
~2~
- 28
an adduct of nonyl phenol with about lO moles nf ethylene
oxide). The latex suspension was pumped through the
membrane using a Sage Instrument Model 341 syringe
pump at a rate of 2 milliliters per minute. The latex
effluent ~as detected by 90 degrees scattering of 537
n~ as measured by a Brice-Phoenix BP 2000 li~ht
scattering photometer using a flow cell.
The latex solution was pumped through the memb~ane until
the light scattering of the effluent began to differ
from that observed for a solution of 0.1% Triton X-lO0
alone, indicating the start o~ penetration of la~ex
particles through the membrane. The latex adsorption
capacity tLAC) was calculated by the following relation:
LAC (mg/cm2) = 10 x V
929.0
where V is the number of milliliters of 0.01 weight
percent latex suspension which could be passed through
the membrane until passa~e of latex was observed.
(c) Bacterial Titer Reduction:
Membranes sterilized by the autoclaving were placed
in a suitable stainless steel filter holder and the
filter holder with membrane in place was exposed to
~ steam at 123 degrees C. for 30 minutes followed by a
6G minute exhaust period in a ~ernitron/BetterBuilt
Century 21 Model 222 laboratory sterilizer. They were
then challen~ed with bacteria at four levels: 106~ 1o
101, 1012 bacteria per square metre .for a total
challenge of about 1012 bac~eria per square metre of
~2~S~
- 29
membrane ~U.OS. P. Bacterial Titer Reduction Test).
The effluent was collected under aseptic conditions
in a sterile glass v~ssel. The number of bacteria
5 in the influent and the effluent was determined by
making serial dilutions of these suspensions and plating
them on a 0.22 micrometer analytical membrane. These
membranes were cultured as Muller-Hinton agar at 38 degrees
C. for 24 hours to grow colonies of Serratia Marcescens
10 and for 48 hours to grow colonies of Pseudo~,onas diminuta.
The colonies growing on the cultured membranes were counted
and the number of colonies observed was assumed equivalent
to the number of bacteria in the solution plated.
As in the case of latex particle adsorption titer
reduction, TR. is defined as the ratio of influent
bacterial count to effll!ent bacterial count:
0 TR = influent count
effluent count.
d), Endotoxin Titer Reduction Test ~lethod:
A 47 D.m , diameter disc of test membrane was pre-
wetted with isopropyl alcohol and placed in a depyrogenat-
25 ed 47 m.u , disc holder of filtration area 0~0009~9m2 ,(0.01 ft2) 9 which had been previously depyrogenated by
heating in an oven at 250 degrees C. for 1 hour. 50ml
pyrogen-free
I
L2~ Sl~
- 30
water was passed through the test membrane and the last
3~4ml were collected in pyr~en-free glassware and
saved as a control for the system~ The membrane was
then challenged with successiv~ 10 ml aliquots of E. Coli
055:B5 purified endotoxin at a flow rate of 5 ml/min/
9~'3 ~: and the effluent collec~ed and saved as above.
The first aliquot was at a concentrati~n of lng/ml~ each
succecsive portion was 10 t;mes the concentration o~ the
previous one up to a ~aximum of 100 mg/ml. The influent
and e~fluent solutions were diluted w;th pyrogen-free
~ater as required and analyzed for the presence of
endotoxin by the Limulus Amebocyte Lysate Test ~United
States Pharmocopeia X~, 1980 page 888).
(e) Resis~ivity of Effluent Water:
Prel;minary Preparation:
Surface modified, microporous, hydrophilic polyamide
membranes were converted to standard cartridge form by
conventional procedures to form cartidges havin~ 0.~
sq. m. (7.5 sq. ft.) of ~iltering area. The cartridges
were then flushed with o.2. molar ammonium hydroxide at
3.5 Kg/sq. cm. (50 psi) pressure across the cartridge
for 6 minutes in order to convert the surface modifyin~
polymer to the hydroxide form. ~he cartrid~es were then
flushed with 1.5 liters of deionized water to remove
- residual
I
~z~s~
3]
ammonium hydroxide and then dried for 12 hours at 79.4 C.
(175 F). The cartridges were then ready for testing
for their ability to deliver high purity water effluent
by the flowing Resistivity Test Method.
s
Resistivity Test.
Water of near theoretical resistivity was generated by
passing tap water through a ~lodel ~ 18090 deionizing
bed (Penfie~lInc~) and through two Unibed iOl- e~change
beds (Culligan Inc.) Test membranes in the form o
standard cartridges were mounted in a cartridge housing
of common design and subjected to a flow of a,pproximately
10. ~ liters per square metre membrane area per minute
using the water from the deionizing system. The effluent
water from the elements was monitored for resistivity
with a Model 3418 conductivity cell (Yellow Springs
Instrument Company), The conductivity cell was connected
to a Model 31 conductivity bridge (Yellow Springs
Instrument Com~)an)r) which allowed the direct measurement
of effluent resistivity as a function of water flow time.
The time in minutes required to reach an effluctlt resis-
tivity of 14 megaohms per centimeter, the generally
accepted water quality limit by the electronics industry,
was determined.
General Method I for the Preparation
By Continuous Casting of the Membranes
of the Followin~ Examples 1-12:
N,ylon 66 resin pellets we-^e cissolvec, in ~o.5~', or~ic acid
~!
~LZO~L5~
- 32
Sufficient activated
Resin R4308, as a 5 weight percent solution in water,
was added to bring the relative proportion of Resin
R4308 to nylon 66 to the desired value. This homogeneous
5 casting solution comprised of (1) the casting resin sys-
tem, i.e., the nylon 66 plu5 R4308, and ~2) the solvent
system, i.e., formic acid plus water, was tested for its
viscosity at 30~ C on a Rion Viscotester with a number
1 rotor (Model VT-04, available from Extech International
10 Corp., Boston, Mass. U.S.A.) operating at 63.8 r.p.m.
The viscosity ~as found to be about 6000 centipoise.
After viscosity testing, the castin~ solution ~as deli~er-
ed by ~etering pump, at flow rates from 250 ~rams per
minute to about 1500 grams per minute, into an in-line
15 mixer of conventional design having a 6.35 cm rotor
whose mixing intensity was controlled over a range of
speeds. Simultaneously a non-sol~7ent, water, was added,
as indicated for each example, to the mixer b~- metered
injeccion to produce the desired ratio of formic acid to
20 water and to induce nucleation of the casting solution
and obtain a visible precipitate.
Upon exiting the mixer, the resulting casting composition
was filtered through a 10 micrometer filter to remove
25 vicible resin particles and was then formed into ~ thin
film on a moving, porous, fibrous, polyester, non wover.
27.3 cm wide, continuous web by a doctor blade with
about ~ 0.02G cm spacing. Within less than 3 seconds the
coated web was immersed into a membrane forming bath of
30 the liquid non-solvent system comprised of a mixture
of formic acid and water, as specified for each example,
for approximately 1 to 3 minutes. The bath concentration
was maintained constant by the con~inuous addition`of
water to the bath at the required rate to compensate for
solvent diffusion
I
~Z~3~ s~
- 33
into the bath from the film of the casting composition.
The nylon membrane so formed on the non-woven, porous
polyester support was washed with water for from about
3 to about 6 minutes to remove residual formic acid.
Excess water was removed from the nylon membrane by
passing it between tensioned rubber wiper blades and the
membrane was wound into suitably sized rolls for storage
or further processing. ~or filtration application or
testing of nylon membrane in flat sheet form, the
membrane sheet was mounted in a restraining frame to
prevent shrinkage in any direction and the membrane was
oven dried to 143.3 degrees C. for about 5 minutes. The
membranes were also converted to filter cartridges by
~nown methods and subjected to testing or filtration
application in cartridge form. All parts and percentages
herein are by weight unless otherwise specified.
Example 1.
The continuous casting method, described under General
Method I above, was employed to produce a surface modif-
;ed, microporous, hydrophilic polyamide membrane. The
casting solution was prepared by mixing about 549 parts
of formic acid, 92.2 parts of a 5 weight percent activated
Resin R4308 in water and 108.1 p~rts of nylon 66 resin
pellets. The mixture was mechanically agitated until
homogeneous.
I
~L2~ 5~3
- 34
The casting solution was pumped into an in-line
mixer operating at 3600 RPM at the rate of 1000 grams
per minute while water was injected into the ~ixer at
the rate of 36.4 grams per minute. The resulting
casting composition was then passed through a 10
micrometer filter to remove visible particles. The
casting composition was maintained at a temparature of
48.5 degrees C. and was delivered onto the polyester
web by means of a doctor blade with an approximately
0.020 cm spacing. The web was passed by the doctor
blade at approximately 15,2 metres per minute and then
into a bath containing 50.1 weight percent fol~ic acid
and the balance water. The membrane was ~hen further
processed as outlined in Method I, that is, it was washed
and then dried in a restraining frame for 5 n~inutes at
140 degrees C. and was then ~eady for filter application
or testing.
Example 2.
The method of Example 1 was repeated but with the
casting solution made up of about 374.5 parts o formic
acid, 53.0 parts of 5.82% activated solution of Resin
R4308 in water~ and 72.5 parts of nylon 66 resin. The
casting solution was pumped at a rate of 500 grams per
minute into an in-line mixer op~rating at 2718 RPM
while water was injected into it at a ra~e of 24.9 grams
per minute. The resulting casting composition was
maintained at 54.0 degrees C. and was cast onto the
~2~: L51~
_ 35
polyester web moving at 10.1 metres per minute. The
web was then immersed in a bath containing 55 percent
formic acid, the balance water, and then further process-
ed as described previously until ready for filter applica-
tion or testing.
Example 3.
The method of Example 2 was repeated except that the
operating speed of the in-line mixer was 2717 RPM, and
that the water was injected into the mixer at a rate
Of 17.9 grans per minute. The casting composition
temperature was controlled at 54 7 degrees C. The
coated web was passed into a bath containing 55.4 weight
percent formic acid, balance water, at a rate of 10.1
metres per minute. ~he membrane was then further
processed as previously described until ready for filter
~plication or testing.
Example 4.
A casting solution composed of about 73 2 weight
percent formic acid, 12.3 weight water, 14.02
weight percent nylon 66 and 0.43 weight percent activated
resin R4308 was prepared by hereinbefore described
procedures. The casting solution was pumped into the
in-line mixer, operating at 3600 RPM, at the rate of
1000 gramC per minute. Water was injected into the mixer
~2~
36
at the rate of 32.9 grams per minute and the resulting
casting composition was maintained at a temperature of
47.6 degrees C. The polyester web was passed by the
doctor blade with a 0.02Q cm spacing at the rate of
16. ~ metres per minute and into the bath containing 50.1
weight percent formic acid, balance water. The membrane
was then further processed as hereinbe~ore described
until ready for filter application or testing.
Example 5.
The membrane forming process of Example 4 was repeated
except that the relative weight nylon 66 and Resin
R4308 was adjusted to 2 weight percent Resin R4308
based on the nylon 66, i.e., there were 2 parts of
Resin R4308 for 100 parts of nylon 66. The casting sol-
ution was delivered to the mixer at the rate of 1000
grams per min~te and the resulting casting composition
temperature wa~ maintained at 47.5 de~rees C. The
web, moving at 16. 8 metres per minute, was immersed in
a bath containing 49,6 weight percent formic acid,
balance water. The membrane was then further processed
as hereinbefore described until ready for testing or
filter application.
Example 6.
The membrane forming process of Example 4 was repeated
except that the ratio of Resin R4308 to nylon 66 in the
~%~
- 37
casting solution was adjusted to 1.5 weight percent, i.e.,
1.5 parts Resin R4308 to 100 parts nylon 66. Water was
introduced into the in-line mixer at the rate of 28.7
grams per minute and the casting composition temperature
was maintained at 47.1 degrees C. The web, moving at
18. 3 metres per minute, was immersed into a bath contain-
ing 50.1 weight percent formic acid and the balance water.
The membrane was then further processed as previously
described until ready for testing or filter application
Example 7
The membrane forming process of Example 4 ~as repeated
except that the ratio of Resin R43L8 to nylon 66 was ad-
15 justed to 1.25 weight percent in the casting solution,
i.e., 1.25 parts Resin R4308 to 100 parts nylon 66.
Water was introduced into the mixer at 28.7 grams per
minute and the casting composition temperature was
maintained at 48.0 degrees C. The web, mo~ing at 16.5
20 metres per minute, was immersed into a bath contaîning
so.2 weight percent formic acid, balance water, and then
further processed as previously described until ready
for testing or filter application.
25 Example 8.
The method of Example 1 was repeated, but with the
casting solution made up of about 487 parts of formic
acid, 58.9 parts of a 2 weight percent activated solution
~2~S~
_ 38
of Resin R4308 in water, 9.9 parts of water and 94,0
parts of nylon 66 resin. The casting solution was
pum~ed at a rate of 500 grams per minute into an in-line
mixer operatin~ at 2648 RPM while water was injected
into it at a rate o~ 25.3 grams per minute. The
resulting casting c~mposition was maintained at 53.7
degree C and cast onto the polyester web movin~ at 10. 4
metres per minute. The web was then immersed into a
bath containing 54 percent formic acid, the balance
1~ water, and then further processed as previously described
until ready for filter application or testing.
Example 9.
The method of Example 8 was repeated except that the
speed of the in-line mixer was 2631 RPM and water was
injected into the mixer at a rate of 12.0 grams per
minute. The resulting casting composition was maintained
at 55.9 degrees C. before casting onto the polyester
web moving at 9. 1 metres per minute. The web ~as
then i~ersed into a bath containing 54 percent ormic
acid, the balance water, and then processed as herein-
before described until ready for ilter application or
testing.
Example 10.
The method of Example 8 was repeated except that the
speed of the in-line mixer was 2719 RPM and water was
15~
- 39
injected into the mixer at a rate of 4.4. grams per
minute. The resulting casting composition was maintained
at 5~7 degrees C. and cast onto the polyester web
moving at 6.7 metres per ninute. The web was immersed
into a bath containing 54 percent formic acid, the
balance water, and then further processed as hereinbeore
described until ready for filter application or testing,
Example 11.
The method of Example 8 was repeated except that the
speed of the in-line was 2651 RPM and water was injected
into the mixer at a rate of 6.5 grams per minute. The
resulting casting composition ~as maintained at 57.4
degrees C and cast onto the polyester web moving at
7.9 metres per minute. The web was immersed into a bath
containing 54 percent formic acid, the balance water
and then fur~ er proc~ssed as hereinbefore described
until ready for filter application or testingO
Example 12.
The method of Example 2 was repeated except that the
speed of the in-line was 2719 RPM and water was injected
into the mixer at a rate of 7.5 grams per minute, The
resulting casting composition was maintained at a temp-
erature of 57.3 degrees C. and cast onto the polyester
3(~
web moving at 6.7 metres per minute. The web was immers-
ed into a bath containing 55 percent formic acid, the
balance water, and was then further processed as described
hereinbe~ore until ready for filter .Ipplication or
testing.
The pore diameters of the membranes of Examples 1-12
and a Control membrane, prepared from nylon 66 without
added membrane surface modifying polymer, were
determined by ~L measurement as described in U. S. Patent
~pecification 4,~,479 with the results set out in Table
1 belo~. The zeta potentials and adsorption capacities
of the membranes were also determined, by tests (a)
and (b) above respectively, with the results set out in
Table 1.
TABLE 1
Adsorption Capacity
Added Weight Percent Calculated Zeta Potential For 0.038 Micrometer
Membrane of Resin R4308 Relative Pore Dlameter In Millivolts Latex In Milligrsms
Example To Nylon 66 Resin Micrometers At pH = 7. 5 Per Square Metre~
4.1 0.1 +15 1033
2 4.2 0.2 +22 1141
3 4.1 0.45 +19 936.
4 3.0 0.1 +13 743
2.0 0.1 +13 ~36
6 1.5 0.1 +12 377
7 1.25 0.1 +13 56
8 1.25 0.2 +11 226.
9 1.25 . 0.8 +18 312
1.25 3 +12 86 ~l~
11 1.25 102 +16 24~
a2 4,2 1.2 +25 452.
Control None 0.1 -20 0
~2t~a~15~3
- 42
The data in Table 1 above demonstrate that the present
process of preparing surface modified, microporous,
hydrophilic membranes produces membranes with positive
zeta potentials in alkaline pH. Furthermore, the data
5 in the table demonstrate that membranes with widely
differing physical pore diameters can be prepared by
this process. Moreover 3 the listed adsorption capacities
for 0.03~ latex spheres, particles whose diameter is
much smaller than the pore size (pore diameter) of
10 these membranes, demonstrates the greatly enhanced
particulate removal efficiencies of these membranes comp-
ared to the control membrane, a microporous, hydrophilic
nylon 66 membrane made b~ the process of U. S. Patent
Specification number 4,340,4/9 Conseq~ently, the
15 membranes are superior to unmodified membranes in ultra-
fine filtration applications.
The membrane of Example 12 was also tested for its ability
to remove the bacterium Serratia marcescens from aqueous
20 suspension by the previously described Bacterial Titer
Reduction Test Method (test (c~ above). For comparative
purposes, an identical membrane, but prepared without
~he added membrane surface modifying polymer, was
included in the test evaluation and is designated as
25 Control in Table II.
- 43
TABLE lI.
Serratia Marcescens Challen~e
Membrane of Challenge Organisms Titer
Example Per Square Metre Membrane Reduction
12 1ol2 5.5 x 106
Control lol2 ~.5 x 10
The results in the above table demonstrate the greatly
enhanced (by nearly 100,000 fold) bacterial removal
efficiency of filter membranes prepared by the addition
of the modifying polymer when compared to a similar
Control Membrane but without surface modification.
The membrane of Example 3 was tested for its ability
to rem~ e E. Coli endotoxin from aqueous suspension by
the previously desc~ibed Endotoxin Titer Reduction lest
Method (test (d)). This endotoxin is believed to be of
molecular dimensions and exis~ in rod forms of abou~
0.001 micrometers in diameter. For comparative purposes,
a similar membrane prepared without the added membrane
surface modifying polymer was included in the test
evaluations and is listed as Control in Table III.
TABLE III.
25 Membrane E~ Coli Endotoxin Concentration, in
of Nanograms Per Milliliter, Required
Example For Positive Effluent.
3 100,000
Control
~ 2~
4~ -
Surprisingly, the membranes in accordance with the
F.resent invention show extremely large improvements in
the removal efficiency of bacterial endotoxins when
compared to unmodified mem~ranes. The presence of low
levels of membrane surface modifying polymer produces
about a 100900Q fold increase in the endotoxin removal
efficiency of the membrane.
Unexpectedly, in addition to being able to remove unwanted
materials of biOlogical activity, the membranes are able
to decrease the adsorptive removal of certain desirable
components of filterable pharmaceutical preparations, For
example, membranes prepared by the method of Exa~ple 2
were tested for their ability to pass a solution of ben-
zalkonium chloride, a commonly used preservative inpharmaceutical products, without undue reduction in the
concentration of this substance. An aqueous solution
of 0.004 percent by weight of ben~alkonium chlorlde
was passed through two layers of 47mm diameter discs
at a rate of 0.7 litres per minute per 92~ cm2 and
the concentration of the preservative in the effluent
relative to that of the influent was determined as a
function of throughput volume. For the purpose of
comparison~ a commerical nylon 66 membrane of the same
pore size rating, designated here as Control, was
similarly tested.
- 45
TABLE IV.
Throughput Required to
Obtain Percent of Influent
Membrane Pore Size ('oncentration (Liters
5 of Example (Micrometers) ~er 929 cm2)
90% 95%
2 0.2 1.5 3.2
Control 0.2 4 5
The data in Table IV above illustrates that the effluent
of filter membranes reaches acceptable levels substant-
ially before the effluent of the Control. This is of
great benefit when filtering such pharmaceutical prepar-
ations because there is less wasta~e of the required amount
f preservative.
General Method II: Preparation
By Batch Procedure of the
Membranes of Examples 13 and 14:
In the following e~amples, polyamide membranes were
prepared containing different membrane surface modifyin~
polymers using the following batch procedure. Membrane
casting resin solutions were prepared by dissolving nylon
25 66 resin pellets of the same nylon 66 as used in Examples
1 - 12 ( or other polyamide as specified in the examples)
in a solution of formic acid and the desired surface
modifying poly~er. Dissolution took place with stirring
LS~
- 46
at about 500 RPM in a jacketed resin kettle maintained
at 30 degrees C. When dissolution was complete (usually
within 3 hours), a non-solvent, water, was added to the
solution in an amount sufficient to adjust the final
concentration of materials to that given in each example.
The water was pumped in at a rate of about 2 ml/min
through an orifice about 1 mm in diameter located under
the surface of the solution at a point about lcm from
the stirring blade. Stirring was ~,aintained at about
500 RPM during addition of the water to induce nucleation.
The casting composition was filtered through a 10 micro-
meter filter, after which about 40 grams of the
resulting casting composition ~as spread out onto a
clean glass plate by means of an adjustable gap
doctor blade. The film was then promptly immersed in a
bath containing formic acid and ~ater in the amounts given
in the ~xamples below.
The membranes were held immersed in the bath ~or several
minutes and were then stripped from the glass plate.
The membranes were washed in water to remove residual
formic acid and were oven-dried for 15 minutes at 96
degrees C. while restr~ined in a fr~me to prevent
shrinkage. The ~lat membrane sheets were then used for
filter applications or for testin~.
~o~
- 47
Example 13.
A membrane was prepared according to General Method II
with the surface modifying polymer being Polycup (register-
ed Trade Mark) 1884, a polyamido/polyamino-epichlorohydrin
as described above having (1) a specific gravity of 1.12 and
(2) 3 viscosity of 325 centipoise as a 35% aqueous solut-
ion. The casting solution contained about 74,2 weight
percent forTnic acid, 10.0 percent water, 14.3 weight
percent nylon 66 and 1.43 weight percent Polycup;
(registered trade mark) 1884~ The casting composition
was spread as a film 0.038 cm: thick on a glass plate
and irrlrnersed in a bath containing 54% by weight fol~ic
acid, the balance being water, The membrane ~as then
further processed as described above under General ~lethod
II.
The membrane prepared was instantly wetted upon contact
with water (less than 1 second) and had a pore size of
about 1 micrometer, as determined by KL measurement.
The zeta potential of the mernbrane was found to be +2.8
mV at a pH of 8Ø
Example 14.
A membrane was prepared accordin~ to General Method II
with the polyamide resin being poly(hexamethylene
azelearnide),(nylon 6,9), and the surface modi~ying
L5~3
- 48
polymer being activated Resin R4308. The casting solution
contained about 65 4. weight percent formic acid, 17.7
weight percent water, 16.0 weight percent nylon 6,9
resin and 0.8 weight percent R4308 resin. The casting
composition was spread as a film 0.053 cms thick on a
glass plate and was immersed in a bath containing 60
percent by weight formic acid, the balance being water
The membrane was then further processed as described above
under General Method ~T,
The membrane of Example 14 was completely wetted immed-
iately upon contact with water (less than 1 second)
and had a pore size of ~ micrometers as determined by
KL measurement. The membrane had a zeta potent;al of ~-3
m~l at a pH of 8Ø Thus, the membrane prepared by this
method was microporous, hydrophilic and exhibited a
positive zeta potential at alkaline pH.
The continuous membrane preparation procedure (General
Method T described above) was used to prepare a number
of membranes, each with a pore size of 0.1 micrometer
and containing various levels of added Resin R430S. The
preparation of ~hese membranes is described in Examples
1,4,5,6 and 7. The membranes were prepared under ident-
ical conditions from casting solutions containing fromabout 4 weight percent added Res;n R4308 to as li~tle
as about 1 weight percent. A similar membrane, but
without added Resin R4308 was also prepared by the process
- 49
of U. S. Patent Specification ~'P 4,340,479 as
a comparative example and is designated as Control in
the discussion below.
5 These membranes were tested for their zeta potential at
pH = 7.5. All of the membranes prepared with added
Resin R4308 (from 1.25 to 4.1 weight percent) had a
strongly positive zeta potential. The Control membrane,
prepared without added Resin R4308, had a strong negative
lO zeta potential under the same measurement conditions.
Thus, even low levels added Resin R4308 were found to
produce membranes with strong positive zeta potential
and improved filtration efficiency toward negatively
charged particulates in aqueous suspension.
The membranes of Examples 1,4, 5,6, and 7 were also
converted into filter cartridges by methods known in the
art. These Filter cartridges were then flushed with
o.2. molar ammonium hydroxide at 3.5~ kg per square cm
20 pressure across the cartridges for a period of 6 minutes,
followed by flushing with 1.5 liters of deionized water.
They were then dr;ed for 12 hours at 79.4 degrees C. The
filter cartridges were then tested for their ability
to deliver, within a short time onset of filtration, high
~5 purity effluent water of extremely low ionic content, a
reguirement for the filtration of electronics grade water.
For comparative purposes, a filter cartridge containing
the Control membrane, prepared under similar conditions
but without the added surface modifying poly-
(~'~ after)
~20~ 5~3
- 50
mer, was included in the test evaluation and is designed
Control in Ta~le V. The times for the effluent of these
filter cartridges to reach a resistivity of 14 megaohms,
as measured by the Resistivity Test described, above,
along with the zeta potentials and particulates adsorption
capacities for the filter membranes are also listed in
Table V.
TABLE V.
Adsorption Cspacity of Tlme in Mlnutes for
Welght Percent 0.038 Micrometer Latex Effluent ~o Reach 14
Membrane of R4308 Added to Zeta Potential in Spheres in Milligrams Megaohms Per Centi-
Example , Nylon 66 Resin Millivolts at pH=7 Per Square Metre Meter Resistivity
1 4.1 +15 1033 22
4 3.0 ~13 743 15
2,0 +13 936 7.5 '
6 1.5 +12 377 2.
7 1.25 +13 560 2,5
Control None -20 0 2,5 C~
~2~
- S2
The results in the table show that the membranes
have novel propert;es useful in electronics water filtra-
tion when compared to prior art membranes. The present
surface modified membranes have positive ~eta potentials
in alkaline media, vastly improved removal efficiencies
for ultrafine particulates and the ability to deliver
purified effluent of extremely low ;onic content in
rapid fashion after the onset of filtration.
The relationship between (1) the time interval, from
the onset of filtration, required to produce the
re~uired filtrate water resisitivity of 14 megaohms/cm
~rinse up time as designated in the ~esistivity Test
described above~ and (2) the percent added Resin R 4308
is shown in the sole Figure; a plot of rinse up time
versus percent added Resin R4308 for each of the above
filter cartridges. The Figure illustrates that the
rinse up time diminishes linearly with decreasing levels
of added Resin R4308. The data in Table V show that a~
about one to one and one-half percent added Resins R4308
the resulting membrane has a strong positive ~eta
potential and a rinse up time substantially identical
to that of an unmodified membrane. This behavi~ r is
highly desirable since this membrane delivers high
resistivity water efficiently and yet provides enhanced
filtration efficiency through electros~ic effectsO
5~
~ 53
ll'LE 15
A surfàce modified, microporous, hydrophilic polyamide
membrane prepared according to the process of tl-e subject
invention from the nylon 66 as used in Examl~les 1-12 and
Resin R430S and having a pore rating of 2 micrometers was
tested for its ability to remove haze and haze precursors
from commercial cherry brandy comprised of 40 percent
alcohol by volume. Prior to filtration, the brandy was
chilled to about 0 de~rees C. at which tem~erature it had
a distinct, turbid appearance, indicating the presence of
an insoluble, dispersed phase of finely divided hazeO The
test was carried out by passin~ the chilled cherry brandy
tllrough a filter media comprised of two layers of the
microporous membrane described above, That is, a disc, ~i7
millimcLers in diameter, and comprised of tt~o layers of
the membrane described above was mounted in a membrane
holding device and the chilled cherry brandy was then
passed througll tl~is filter medium at a rate of 0.5 liters
minute per 929cm (per square foot~ of membrane surface
area.
~e initial pressure drop across the filter medium was
0.3 2 kg/sq.cm (4.5 psi). ~fter 5 hours onstream, the
pressure drop had increased to 0.51 Itg~/s~,cm.(7.2 psi).
Over the 5 hours of filtration~ the filter effl~lent had a
crystal clear appearance, without evidence o:E any haze,
l'he total volume of cherry brandy filtered over the 5
hour period corresponded to 125 liters per 929 cm2 (per
square foot) of filter medium. After filtration, the
cherry brandy was allowed to warm up to ambient temper-
ature. No haze developed. Further, even after recoolin~,
the cherry brandy remained haze freeO
This Exa~ple demonstrates that a membrane of the subject
_ .,
.,.
.
:~2~
-- 5~!
invention is useful for treating alcoholic beverages to
render them haze free and stable against haze forrnation.
Example 16
To further demonstrate the ability of the membranes
in accordance with the subject invention to ~perate in a
clean manner 2S required in certain filtrati~n applications,
such as the manufacture of near theoretical resistivity
water for electronics manufac~ure, a series of elements
A-D, as described below, were tested for"extractables"
by the process described below,
In this series of tests, corrugated filter elements of
conventional design having an effective surface area of
about 0~46 squ metre ~5 square feet) were prepared by
conventional means rom three different microporous
membranes. These elements, labelled A through D in Table
VI below, were prepared from membranes which themselves
were initially prepared by the processes indicated below:
Element
A ~ hvdrophilic, microporous polyamide membrane
with a pore rating of 0,2 microme~ers was
prepared b!~ the general process described in
2~ UOS~ Patent specification ~i,340,479 from the
sa~ne nylon 66 as used in Examples 1-12. The
formed membrane was coated with Resin R4308
by impregnating the membrane with a 3 percent
by weight solution of R4308 in water,
the Resin R4308 having been a~ti-
~ ,.~
`~
- 55
vated according to the manufacturer'~
recommendati~ns, ~llowing which the
membrane was wiped t~ remove excess
res;n and thereafter formed into the
filter element denoted as element A
in Table YI below.
B ~he membrane of element B was pre-
pared by the same process as des-
cribed above with regard to the
membrane of element A. This mem-
brane also had a pore rating of 0.2
micrometers.
C This membrane was prepared by the
cocasting process of the subject
invent;on from (1) the same nylon 66
as ~he membranes of elements A and B
above and (2) Resin R430B. The
resulting membrane also ha~ a pore
rating of 0.2 micro~eters and was
comprised o~ 98 percent polyamide
and 2 percent by weight Resin R430B.
~5 D ~Control) This Control membrane was prepared
by the process described in U. S.
~atent 4,340,479 from the same nylon
66 as elements A and B above. The
resulting membrane alsc had a pore
rating of 0.2 micrometers~ It did
not contain a mr~ifying polymer as
e~ther (l) an integral part of the
~ 6tructure (as ~id element C) or (23
a coating comp~nent of the membrane
(as in the case o~ elements A and B~.
- 56
The filter elements A through D as described above were
tested as follows:
Elements A and B were each (separately) subjected to a
leaching step by passing 1.8962 litres (0.5 U.S.gallons)
per minute per element of room temperature, deionized water
through the respectiveelement for the time specified in
Table VI below. This leaching step ~as carried out in an
effort to remove as much soluble material from elements A
and B as possible. Neither element C nor element ~ was
give the benefit of this treatment~
After the water leaching step carried out with elements A
and B, each of the elements was individuallv subjected to a
room teDperature, deionized water flush at the pressure and
for the time specified in Table VI below. Note that the
flushing times and pressures resulted in a total flow of
~ater through each individual element of about 189
litres (~0 U.S.gallons) (elements A and B) and 227
litres (60 U~S. gallons) (elements C and D)~
After the deionized water flushing step, the elements were
dried at 96. C (205 degrees F) for about 12 hours,
autoclaved with steam at 121 degrees C. for about 1 hour
and then extracted(again, each element separately) with
deionized water, The extraction step was carried out by
plugging the bottom of each element and then placing each
element in its own bath of one and one-half litres of
deionized water, following which each of the elements
was reciprocated in an up and down manner (with the top of
the filter element rising about ~ centimeters above the
upper level of thebath on the up-stro~e) for 4 hours.
In each case, the water in the bath was then evaporated and
the non-volatile residue remaining behind weighed to deter-
a~ s~
- 57
mine the extractable material in each filter element~ The
values are set out in Table VI below.
TA~LE VI
Deion- Deion- Extract- Zeta
ized ~ater ized Water able Mat- Poten-
Element Leach Flush erial (mg) ti~ (mv)
A 30 ~in. 5 minD at 96 18-20
1.4 kg/sq.
cm (20 psi)
B 60 min. 5 min, at 68 18-20
1.4 kg/sq~
cm (20 psi)
C None 3 min, at 27 18-20
3.5 kg/sq,
cm.(50 psi)
D None 3 min. at ~5 -18
3~5 kg/sqO
cm.(50 psi)
As can be seen from Table VI, elements C and D had
substantially reduced extractables compared with the
elements prepared from coated polyamide membranes (A and
B)o This was the case e~en though ele~ents A and B were
given a deioni~ed water leach (of 30 and 60 minutes
respectively). As can also be seen from Table VI, element
C, prepared by the cocast process in accordance with this
invention, had a low level of extractables comparable
to the Control element D. However, element C combines
- 30 the desirable positive zeta potential at pH 7 (as well as
at higher pH levels) with the low extractables of the
Control D which has, for many purposes, the undesirable
negative zeta potential at pH7(and at hi~her pH values
as well)~
~3~ 5~3
5$
Industrial Applicability:
The surface modiEied membranes hereinbefore described h~ve
been demonstrat~d to be superior in many important filtr-
5 ation related properties to untreated prior art membranes.They are also superior in many respects to coated memb-
ranes, e.g.~ in improved efficiency in utilization of
the surface modi~ying polymer and in certain surface
properties of the comparative end products. They car.
10 be used for filtering applications in their manufactured
form, with or without the incorporation of the substrate
upon which they are found. Two or more membranes can
be combined or secured to each other to form mul~iple
layer membrane filter sheets or they may be converted
into filter elements by kno~m n-ethods and emplo)ed in
filter cartridges, e.g., as filter elements in the form
of a corrugated sheet supported within a conventional
cartridge.
20 The membranes display positive zeta potentials over a
broad pH range of fr~m about 3 to about 10 and show
greatly enhanced removal efficlencies toward negatively
charged particles in aqueous suspension. Furthermore,
they have enhanced ef~iciency to remove bacteria and
endotoxins from aqueous fluids. Moreover, the irnproved
physical and chemical properties~coupled with their
ability to quickly deliver high purity effl~ent water,
free from microparticulate and ionic contamlnantsg makes
them particularly desirable for use in microelectronics
- s ('
manufacture,
These me~branes find use in industry and the medical
field for treatment of wcter supplies for critical
applications such as water for injection into humans,
in microelectronics manufacture for the reasons
discussed above, for the filtration of blood serum to
help achieve sterility, for filtration of parental
fluids, and generally for any use where c~ ion containing
fluid must be filtered to a high degree of clarity~
