Note: Descriptions are shown in the official language in which they were submitted.
3~6~i;
FILTER ELEMENT HAVING MICROPOROUS
.. . .
FII.TER MEMBRAME
1. FIELD OF THE I~VE~T-LON
This invention relates to filter elements utilizing
hydrophilic microporous membrane as the filtration media, and
more particularly to filter elements utilizing cylindrlcal
pleated membrane, said filter elements being suitable for the
filtration of aqueous fluids, in particular parenteral or
body liquids.
2. PRIOR AR~
In many applications it is necessary to totally remove
particles having dimensions in the submicrometer range. For
this purpose, it is well known in the art to use a thin
polymeric layer that is rendered highly porous with a substan-
tially uniform pore size. Such layers are commonly termed
microporous filtration membrane.
One characteristic of suc~ microporous filter membrane is
that they are extremely fragile and easily rupture when
subjected to deformation due to rough handling, bending, or
fluid pressure~ ~ince even the most minute crack or break
will destroy the effectiveness, it is necessary to use extreme
care in manufac-ture and use.
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Microporous filter membrane find many uses in industry,
science and education. A common industrial application is the "cold"
sterilization of phGrmaceuticals ~nd the stabilization of alcoholic
beverages. In cold sterilization, the membrane has a sufficiently
small pore size to block the passage of all bacteria present in the
unfiltered fluid supplied to the upstream side. In the production of
alcoholic beverages, the removal of bacteria, yeast and molds,
st~bilizes and clarifies the beveraae. In the production of phar-
maceuticals, the removal of bacteria is cn essential step for obvious
health reasons. In all of these applications it is essential that the
filter membrane used be hydrophilic in order to filter such aqueous
f luids.
There are many types of filter membranes available and
processes for producing such membrane.
Nyion microporous filter membrane is well known in the art, for
exarnple, U.S. Patent No. 3,876,738 to Marinaccio et al (1975)
describes a process for preparing nylon microporous membrane 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 published in 1979 describes a similar type process
for producing nylon membrane.
Other type polymeric microporous membranes, including nylon
and processes for producing such membranes are described, for
example in the following U.S. Patents:
3,642,668 to Bailey et al (1972);
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4,203,847 to Gr ndine, II (1980),
4,203,848 to Grandinel II (1980), and
4,247,498 to Castro, (1980).
Commercially available nylon microporous filter membranes
are available from Pall Corp., Glencove, N.Y., under the trade-
mark ULTIPOR N66 and N66 POSIDYNE. Another commercially signi-
ficant filter membrane made of polyvinylidene fluoride is
available from Millipore Corp., Bedford, Massachusetts, under
the trademark DURAPORE. This membrane is probably produced by
the aforementioned Grandine, II patents.
Additionally, the Assignee of this application i5 selling
cationically charged modified nylon microporous filter membrane
under the trademark ZETAPOR. These membranes are described and
claimed in U.S. Patent No. 4,473,475 to Barnés, and U.S. Serial
~o. 314,307 filed on October 23, 1981 to Ostreicher et al, now
U~S~ Patent ~o. 4,473,474. Barnes et al describes the use of
charged modified membrane for the filtration of high purity
water (18 megohm-centimeter resistivity) used in the electronics
industry, and Ostreicher et al describes the use of charged
modified membrane for the filtration of parenteral or body
liquids. Additionally, it should be noted that these filter
membranes are typically reinforced by various means. A unique
method of reinforcement is described in the Assignee's Canadian
Serial ~o. 417,737 filed December 15, 1982 to Barnes et al.
~2~
All of the aforementioned membranes, besides being used in
sheet form, are used in various type filter elements. Generally, the
filter element comprises the filter membrane and a filter housing
with a seaiing surface in sealing relationship with a sealing area of
the membrane. A well known type filter element is the pleated
cartridge type filter element described, for example, in U.S. Patent
No. 3,457,339 to Pall et al (1969).
Another well known type of filter element is the hermetically sealed
intravenous unit described in U.S. Patent No. 4,113,627 to Leason
(1978).
In the critical applications for such filter elements, it is
imperative that the filter membrane not be damaged during
production and that the filtrate not bypass the filter membrane.
Either situotion could be catastropicl for example, allowing con-
taminants to enter the blood stream of a patientO It is therefore
necessary that an undamaged seal exist between the sealing qrea of
the membrane and the sealing surface of the filter housing to prevent
leakage around the filter membrane. In order to insure such
integrity, the filter element is "integrity tested" to insure the
integrity of the fiiter element. This is generally accomplished by a
"bubble point" test of the filter element by methods well known in
the qrt. A particular type of integrity testing device for filter
cartridges is commerci~lly available under the name ZETAWATCH,
from AMF Cuno Division, ~eriden, Connecticut and described and
claimed in the Assignee's U.S.Patent 4,384~474 to Kowalski. This
g~
integrity tester is self contained and electrically monitors the
individual cartridge elementls inteyrity within a multiple cartridge
housing. Other methods of such integrity testing are described in
"Non-Destructive Test For Bacterial Retentive Fiiters" by Ben
Trasen which was published in the September/October 1979 issue of
the Journal of Parenteral Drug Association, pages 273-2798. All of
the known integrity tests require a thorough wetting of the
membrane and sealing surfaces associated therewith ~o provide qn
accurate determination of integrity. If the filter membrane is
broken, even microscopically, if the membrane is improperly
installed, or if the membrane sealing surfaces are not completely
wetted, bubbles will appear immediately at the point of the break or
leak.
Additionally, any filter element must, particularly when used to
filter parenteral or biological liguids, have a minimum of extractable
contaminnnts introduced into the filtrute. These contaminants may
be harmful toxins when introduced into a patient. Specifical!y, any
filfer element must meet the test standards of the industry, e.g.
ASTM D-3861-79.
Still further, filter elements used to filter parenteral or bio-
logical liquids should be heat sterilizable and autoclavable, without
deterioration or discoloration of the housing or membrune or
deterioration of the seal between the membrane and housing. A
preferred housing nnaterial is polypropylene which is hydrophobic.
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Several methods of sealing filter elements have been employed
in the past. These methods include pressure clamping, heat sealing,
ultrasonic welding, adhesive cmd solvent bonding, and injection
molding. These prior art methods fail to provide on occasion the
100% positive seal which is necessary to prevent leakages. In
particular where microporous filter membranes are used in the filter
element, there is some danger when using these methods that the
delicate filter media will be damaged during the sealing process.
Known methods of pressure clamping and other mechanical
interlocking systerns tend to distort the filter membrane or actually
damage the membrane at the clamping edges, thereby destroying the
integrity of the membrane and al lowing contaminants to pass
through. Also conditions such as time, and heat stress relieving can
allow the pressure seal to relax. Additionally, this method is
particularly complicated when a pleated filter cartridge is assembled.
Known methods of heat sealiny, sonic welding and related thermo-
mechanical bonding methods may also damaae the filter membrane at
the seGling edges. The use of adhesives or solvent bonding has
disadvGntages in that another material is introduced into the filter
element that can lead to extractable contaminqnts. Often the
constituents of an adhesive or solvent system may also damage the
filter rnembrane~
The foregoing methods of seGling the filter housing to a filter
membrane are troubelsome when a hydrophobic sealing surface is in
oontGct with a hydrophilic sealing area. This is often the case when
V'7~i6
the filter element is used to filter biological or parenteral liquids
where it is very highly desirable to use a polypropylene housing,
(which is resistant to autoclaving and heat sterilization) and un-
desirabls to use adhesives or solvents for sealing ~to avoid high
extractables). For such elements, the housing is usually thermo-
plastical Iy sealed to the membrane, increasing the chances for
damage to the sealing areas of the membrane. Additionally, it
appears that the hydrophobic sealing surface of the housing in
contact with the porous hydrophilic sealing area of the membrane
increases the chances that the filter element will not pass industry
integrity tests. This is probably brought about by the incomplete
wetting of the membrane/housing interface which gives a reduced
bubble point. For example, it has been found that in the thermo-
plastic sealing of polypropylene end caps to cylindrical pleated nylon
membrane filter cartridges an unacceptably low percentage of the
cartridges passed the industry integrity test.
More specifically, the following prior art references are
relevant to the invention described and claimed herein.
U.S. Patent No. 1,476,392 to Carrol! (1923) describes a process
of making a composite film by casting a plastic or flowable cellusoic
material, e.g. cellulose acetate, on to a moving wheel from a
plurality of compartments to thereby produce a plurality of adjacent
film strips. This reference does not teach or suggest the production
of a microporous fiiter membrane.
12~
U.S. Patent No. 2,663,660, to La~ (1953) describes a method
of assembling filter elements7 e.g. a filter cartridge, hy producing an
elongated strip of filter paper and folding elongated tapes of adhesive
material on the elongated edges. The filter paper is fhen cut to size
and pleated transversely of the length of the strip, and rolled into an
annulus forrn. The outer portions of the adhesive tqpe material is
then molstened with a suitable solvent material for the adhesive
material of which the tapes are formed and thereafter the ends of
the annulus are capped by end caps. The end caps are preferably
heavy cardboard. ~ does not teach or suggest the use of such a
method in conjunction with microporous membrane nor is such a
method suitable for producing filter cartridges for filtering
parenteral or biological liquids wherein extractables must be
minimized.
UOS. Patent No. 3,û13,607, to Jackson et al (1961) relates to a
method of end cQpping tubular filter elements sf paper, cardboard,
felt, woven tissue, etc. Thermoplastic end caps are subjected to heat
induced in the field of an electric inductance coil in contact with the
cap, to a point where the cap is softened so that the edges of the
filter can be embedded in the cap to the depth required to bind the
parts together. A metallic strip is applied to the edges of the filter
and external support jacket, or the edges of the filter element and
external support jacket are coated with an electrically conductive or
semiconductive material~ so as to reinforce the filter edges, and
enhance the heat conductivity through the edges and end cap.
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Jackson et al does not utilize an organic polymeric microporous filter
membrane and thus does not recognize the problems associated with
the integrity testing of filter elements containing such hydrophilic
membrane in conjunction with a hydrophobic end cap.
Additionally, the use of a metalic strip on the edges of the
Jackson et al filter limits greatly the application to which the end
capped filter elements can be put. For example, under certain
conditions, the metai lic strip can corrode and/or contaminate the
material being filtered or the medium being filtered. Such a filter
element is completely unacceptable for the filtration of biological
and parenteral liquidsO Still further the use of such a metalic strip on
the edges of the filter elements increases the cost of making the
filter elements, and complicates the procedure used in corrugating
such filter elements.
U.S. Patent No. 3,407,252 to Pa!!_et al (1968) describes the
production of a corrugated or pleated filter media in annulus form
which utilizes a ribbon or tape of bonding agent such as a heat
sealable and curable epoxy resin, to form a leak-proof seal along the
longitudinal meeting of the pleated filter media.
U.S. Patent No. 3,457,339 to Pall et al (1969) describes a
process for applying preformed end caps to filter sheet material,
particularly sheet materials formed olF fiber and in substantially
tubular shape. The process involves heating the inside face of the
thermoplastic end cap to fuse a portion of the cap into a liquid. The
liquid is of a viscosity which is capable o~ penetrating through the
3~
pores of the filter sheet. The edges of the cylindrical sheet are then
embedded in the liquified end cap so that the liquidified thermo-
plastic material penetrates through the pores of the embedded
portions of the filter sheet material from one surface to the other.
The liquid plastic is then hardened and said to form~a substantially
continuous leak proof matrix of end cap material permeating through
the pores of the filter material and bonding the filter sheet to the
end cap in a leak proof seal.
This process for applying end caps to a filter sheet has the
advantage in that it does not require the use of adhesives~ If this Pall
et al process, howeverl is utilized using end caps of a hydrophobic
material and hydrophilic membrane, an excessive percentage of the
cartridges do not pass the industry integrity test. It is believed that
this is due to the cartridge not being completely wetted at the
interface between the hydrophilic membrane and the hydrophobic end
cap. Hydrophobic type end caps may be utilized if the cartridge is
integrity tested in a non-aqueous solvent. This7 however, limits the
application of the filter eiement. IF a hydrophilic type end cap, e.g.
polyester, is used, the cartridge will generally have inferior solvent
and chemical resistance and inferior resistance to autoclaving and
heat~
This Pall et a! process requires that the sealing areas of the
filter sheet material be porous to permit penetration of the liquified
thermoplastic material through the pores of the embedded portion of
~L;~7
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the filter sheet material from one surface to the other. Additionally,
during prosecution Pall et al states:
"...The instant process is simple enough to enable rapid
manufactore of filter elements with a minimum of m~nu
facturing steps and without the necessity of employing bonding
agents and components other than the actual m~terials of the
fi!t~...~
In effect Pa!l et al teaches away from Applicant's invention
which utilizes a substantially non-porous sealing area and which may,
in its preferred embodiment~ utilize other components than the
actual materials of the filter and end cap.
U.S. Patent No. 3,471,019 to Trasen et al (1969) describes a filter
unit comprised of a two-part housing provided with sealing portions
adapted to be aligned with each other and with a peripheral portion
oF the filter completely surrounding the central portions of the filter.
In assembly of the unit1 the sealing portions of of the housings are
pressed against the opposite sides of the filter and the sealing portion
of at least one of the parts of the housing is heated to cause the
material thereof to melt and flow through the aligned pores of the
peripheral portions of the filter and fused to the sealing portion of
the other part of the housing~ A similar type filter and sealing
method is described in U.S. Patent No. 3,782,083 to Rosenberq (1974)
wherein the plastic material runs through the pores of the filter
element forming a fluid tight integral seal closing all sides of the
element to fluid flow.
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U.S. Patent No. 3,487,943 to Buckman (1967) describes a filter
elernent made of pleated filter paper. One portion of the filter
element is modified so that in operation of the filter the liquid flow
velocity through the modified portion is less than that through the
remainder of the element. The modified portion may be formed by
compressing together a series of pleats or by sealing to a group of
pleats on one side of the element a sheet of similar or dissimilar
filter material. The similar or dissimilar filter material is sealed to
the annulus cartridge over the inner or outer periphery of the
cartridge and does not form a continuous edge along the top of the
filter near the end cap.
U.S. Patent No. 3~591,010 to Pa!l et al (1971) describes a
corrugated element having a microporous layer deposited on a
substrate sheet provided with portions of reduced porosity at the
areqs of the base folds of the corrugGtions.
U.S. Patent No. 3,815,754 to ~ (1974) describes a box
filter wherein the elements of the filter housing are bonded to the
filter sheet by fused integrqtion of the housing members through the
open pores of the filter element, forming a fluid tight seal all along
the sides of the filter sheet. Such a bond is obtained by, for example9
ultrasonic welcling9 solvent softening or heat fusion.
U.S. Patent No. 3~865,919 and 3,867,294 to Pall et al (1975)
describe cylindrical eiements hqving an improved side seam seal
which c~n be bonded to end caps in a leak type manner.
~ Z(: V'766
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U.S. Patent No. 3,954,625 to h~ichalski (1976) describes a filter
which includes a plastic housing and an intermediate filter screen.
The peripheral portion of the screen is sealed between the two
housing halves by flowing a portion of at least one of the housing
halves through the screen and bonding that portion to the other
housing half~
U.S. patent No. 45101,423 to Merrill et al (1978) describes a
tubular fiitration element whose ends are impregnGted with a suitable
sealing adhesive. When the adhesive material cures, the end portion
provides mechanical support for the tube and blocks the passage of
the fluid or the particulate and bacterial contaminant. Merril! et al
requires that the sealing material used to form the ends must be
hydrophilic when cured, stating:
"If the sealant rendered the filter adjacent to it hydro-
phobic, the filter would not be wetted and would not then offercapillary resistance to the bubble point test gas, therefore the
bubble point could not be used as an indication of filter
integrity..." (Col. 9, lines 59-64).
'llt will be understood that if the outer layer (of the filter)
is formed from a lacquer impregnated paper, the resilient
members can safely apply a sealing force sufficient to block the
fluid fronn the end portions so that a hydrophobic sealing
material may be used.'l (Col. 10, lines 6-10).
The filtration element is supported and sealed within a housing
by radial seal force, i.e. the filtration element and housing are not in
thermoplastic sealing relationship to each otherO
U.S. Patent No. 4,154,688 to Pall (1979) describes the use of
thermoplastic end cap applied to the oper- ends of a filtered tube in
accordance with the aforementioned U.S. patent No. 3,457,39 to Pall
et a!.
U.S. Pctent No. 4,1939876 to Leeke et a! ~1980) describes
dryforming the peripheral portion of discs of f i Iter media,
particularly filter media containing non-compressable particulate to
suppress edge leakage in filter presses.
In assignee's U.S. patent ~o. 4,347,208 to Southall,
a method is described of making a filter cell comprised
of two cellulosic fiber containing filter media having
a-sealed periphery. The method comprises compressing
the periphery of each filter medi~ to form a flange. The media are
then aligned to provide intimate face to face contact betwen the
flanges ~nd a spacer means provided between the media to cause
each to dish outwardly from the other media. The media and spacer
means are then placed into a mold surrounding the flanges. The mold
has a means for providing a recompression force to the inner portions
of the flanges. A thermoplastic polymer is then injected into the
mold to form a seal around the flanges.
Additionally, MICRO-SCREEN filter cartridges are
commercially available from AMF Cuno Division, Meriden,
Connecticut, comprising a stainless steel pleated cylindrical screen
welded to stqinless steel end caps. Shim stock is welded to the
screens qt both ends to effectively seal off the end so that the end
~.
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caps can be welded thereon without destroying the filter screen
thereunder.
In summary, all of the prior art uncovered by applicant relating
to sealing filters, generally requires that the filter media sealing area
be porous, so that when a thermoplastic or sealing surface is applied
thereto it flows through the porous media to effect the seal.
,., ' . ,
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OBJ CTS AND SUMMARY OF THE INVENTION
It is an object of this invention to provide filter element which
has an effective seal between a hydrophilic membrane and the sealing
surface of the filter housing.
It is a further object of this invention to provide an effective
seal without the use of adhesives.
It is still a further object of this invention to provide a filter
element which is p~rticul~rly useful for the filtration of aqueous
fluids, in particular biological and parenteral liquids.
It is yet another object of this invention to provide a filter
element comprising a fragile microporous filter membrane in cylin-
drical form which has toughened ends permitting the ends to be
embedded in an end cap without damage to the ends and/or sealing
integrity of the filter element.
It is still another object of this invention to provide a filter
membrane for use in the filter element of this invention.
It is a further object of this invention to provide novel
processes for producing the filter elements and filter membranes of
this invention.
In accordance with the present invention, a filter element is
provi~ed which comprises:
a) a hydrophilic organic polymeric microporous filter
membrane having a preformed substantially non-porous sealing
~ rea;
,'
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b) a filter housing having preferabiy a hydrophobic
thermoplastic sealing surface in thermoplastic sealing relation-
ship with the sealing area.
Preferably the fiiter membrane is in pleated cylindrical formhaving the non-porous sealing areas at each end of the cylinder and
the housing having an end cap at each end of the cylinder.
The filter membrane used in the aforementioned preferred
filter element comprises an elongated porous filtration area longi-
tudinally bordered by the substantially non-porous sealing areas. This
filter membrane is produced by a novel simultaneous casting and
quenching method which simultaneously produces the filtration area
and non-porous sealing areas. The filter membrane used may also be
produced by preparing the filter membrane by known methods and
then col lapsing the pores along the longitudinal borders of the
filtration areas.
The filter elements of this invention are useful for the filtra-
tion of aqueous liquids, particularly parenteral or body liquids.
o~
_ 1 9_
BRIEF DSCRIPTION OF THE DRAWINGS
Fig. I is u perspective view, pal ~ially broken away, of a
preferred filter element of this invention.
Fig. 2 is a top view, partially in section, of the filter element of
Fig 1.
Fig. 3 is an enlarged view in section, taken along iine 3-3 of
Fig. I depicting the sealing surface between the membrane and filter
element.
Fig. 4, is a schematic perspective of an apparatus that may be
used to prepare a filter membrane by collapsing the pores along the
longitudinal borders of the filtration area.
Fig~ 4 A and B are two embodiments of crushing rollers used in
the apparatus of Fig. 4.
Fig. 5 is a schematic perspective of an apparatus that may be
used to prepare a filter membrGne by simultaneously casting the
filtration area and non-porous sealing arec of the membrane.
.
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C)ETAILED DESCRIPTION OF THE INVENTION
Figs. I through 3 depict a preferred embodiment of the filter
element of this invention. The filter element, generally designated
(lû) is comprised of the filter membrcne (12) and the filter housing,
generally designuted (14). The filter membrane is in cylindrical form
having the substantially non-porous area (16) at each end of the
cylinder (18). Referring to F;g. 3, the filter mernbrane (12) is
sandwiched between inner and outer layers ~20) and (22) of, for
excmple, polypropylene woven netting.
The composite of filter membrane (12) and inner and outer
layers ~20 & 22) is pleated transversely to its length and formed into
cylinder (18). The cylinder (18) is then slipped over a foraminous
cylindrical core (24) which is provided with aperh7res (26) for flow
into the open interior of the core (24). The filter membrane (121 qnd
core (24) are then slipped into an outer cylindrical member (28) which
is also prov~ded with apertures (30). The ends of the cylinders are
then capped by end caps (32 ~ 34).
The end caps (32~34) are sealed by thermoplastic fusion to the
non-porous areas (16) of the filter membrane (12~. The end caps
(32&34) close off the interior from the exterior of the filter element.
The fluid can thus flow from the outside to the interior of the filter
element, since interior and exterior are completely separated by the
filter element and sealed off by the end caps (32&34). The end caps
(32&34) each have a central aperture (36&38).
. .
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The preformed end caps (32 & 34) are preferably applied to th,e
cylindrical membrane (18) by heating an inside face of the thermo-
plastic end cap to a temperature sufficient to soften and preferabl~r
not liquify9 a sufficient amount of the end cap to form a thermo-
plastic seal with the non-porous area at each end of the cylinder All
oF the edges of one end of the cylinder are them embedded into the
softened end cap. The softened end cap mcterial is then hardened,
typically by qmbient conditions, to form a thermoplastic sealincl
relationship between the sealing surface of the end cap and nonporous
area thereby forming a leak proof seql.
A method of applying end caps to filter elements is described in
the aformentioned U.S. Patent No. 3,457,339 to Pall et al. Such a
method and apparatus described therein may be modified to apply end
caps in this invention. The major differences between the method
used in this invention and the Pall et al method, is that Pall et al
liquifies a portion olF the end cap which permeates through the porous
sealing surface of the fiiter membrane; whereas Applicants
preferGbly do not require liauefaction of the sealing portion of the
end cap because the séaling surface of the membrane is non-porous.
End caps of thermoplastic materials are preferred because of
the ease of bonding, but it is ~Iso possible to use thermosetting resins
in a thermoplastic~ fusable or heat softenab!e stage of poly-
merization, until the bondings have been effected, after which the
curing of the resin can be completed to produce a structure which
can no longer be separated. Such a structure is autoclavable without
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danger of destroying the fluid tight seal between the housing portions
and the filter membrane and the end caps. Thermoplastic resins
whose softening point is sufficiently high so that they are not
softened under sterlizing autoclaving conditions are preferred for
medical use. Exemplary of the plastic materials which can be used
are polyolefins (polyethylene, polypropylene, polybutylene, polyiso-
butylene), polyamides, polyvinylchloride, polyvinylidene chloride,
polyacrylonitrile, polyesters, polycarbonates, polymethacrylate, poly-
aallyl9 and polyoxymethylene resins. Polytetrafluorethylene and
polytrifluorochloroethylene qlso can be used. Polypropylene is
preferred for the filtration of biological liquids in that it can
withstcmd autoclaving and sterlizing without discoloration or
distortion. Other type materials, which may be hydrophilic are
generally unsuitable for such uses due to discoloration9 distortion,
etc., however they may be used in conjunction with the membrane of
this invention for other uses.
The hydrophilic organic polymeric microporous filter mem-
branes used in the filter element of this invention are well known in
the art.
By the use of the term "microporous membrane" as used herein,
it is meant a porous single layer, multiple layer or reinforced single
or multiple layer membrane, having an effective pore size of at least
.I microns or larger or an initial bubble point ~IBP), as that term as
used herein, in water of less than 9û psi. A maximum pore size used
for such membrane is preferably about 1.2 microns or an IBP or
~ .. .- -
.
, .
.
greater than 8 psi. Preferably, but not necessarily, a single layer
membrane is substantial Iy symmetrical and isotropic. By
"symmetrical", it is meant that the pore structure is substantially the
same on both sides of the membrGne. By the use of the term
"isotropic", it is mennt the membrane has a uniform pore structure
~hroughout the membrane.
The microporous membranes used in fhis invention are hydro-
philic. By the use of the term "hydrophilic", in describing the
rnembranes, it is meant a membrane which adsorbs or absorbs water.
Generally, such hydrophilicity is produced by the presence of a
sufficient amount of hydroxide (OH~) carboxyl (-COOH), amino (NH2)
and/or similar functional groups on the surface of the membrane.
Such groups assist in the adsorption and/or absorption of the water
onto the membrane, i.e. "wetting out" of the membrane. Such
hydrophilicity is preferred in the filtration of aqueous fluid.
Preferred microporous membranes are those produced from
nylon. The term "nylon" is intended to embrace film forming
polyamide resins including copolymers and terpolymers which include
the recurring amido grouping.
While, generally, the various nylon or polyamide resins are
copolymers of diamine and a dicarboxylic acid, or homopolymers of a
lactam and an amino acid, ~hey vary widely in crystallinity or solids
structure, melting point, and other physical properties. Preferred
nylons for use in this invention are copolymers of hexamethylene
diamine and adipic acid (nylon 66), copolymers of hexamethylene
; ,
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diamine and sebacic acid (nylon 610), and homopolymers of poly-o-
caprolactam (nylon 6). Alternatively, these preferred polyamide
resins have n ratio of methylene (CH2) to amide (NHCO) groups
within the range about 5:1 to about 8:1, most preferably about 5:1 to
about 7:1. Nylon 6 and nylon 66 each have a ratio of 6:1, whereas
nylon 610 has a ratio of 8:1. The nylon polymers are available in a
wide variety of grades, which vary appreciably with respect to
molecular weight, within the range from about 15,û0û to about 42,ûû0
(number average molecular weight) and in other characteristics.
The highly preferred species of the units composing the polymer
chain is polyhexamethylene adipamide, i.e. nylon 66, and molecular
weights above about 30,000 qre preferred. Polymers free of additives
are generally preferred, but the addition of antioxidants or similar
additives may have benefit under some conditions.
Additionally, any of the hydrophilic type microporous mem--
branes used in commercially available filter elements produced by
numerous companies9 are potentially suitable for use in the filter
elements of this invention, for example, Pall Corp.'s N66 ULTIPOR
and POS!DYNE N66 (nylon), Millipore's DURAPORE (polyvinylidene
fluoride), Gelman Sciences Inc.'s METRICEL (esters of cellulose, PVC
copoiymer) VERSAPOR (acrylic copolymer), Ghia Corp's nylon
membrane.
The preferred microporous membranes are produced from nylon
by the method disclosed in U.S. Patent No. 3,876,738 to Marinaccio
et al~ Another method for producing such membranes is described in
the aforementioned published European Patent Applica-
tion No. 0 005 536 to Pall.
Both of these methods for producing nylon microporous mern-
branes may be described as "quench techniques"; i.e. casting or
extruding a solution of a film forming polymer on70 a substrate and
quenching the cast film.
Broadly, Marinaccio et al produces microporous membrane by
casting or extruding onto a substrate a casting solution oF a film-
forming polymer in a solvent system and quenching in a bath
comprised of a nonsolvent system for the polymer The most
important parameter responsible for development of micropores in
the film (e.g. pore size) according to _arinaccio et al, is the solvent
system employed with the polymer and the nonsolvent system used in
quenching the film. The selection of the solvent for the polymer is
deterrnined by the nature of the poiymer materiql used and can be
empirically determined on the basis of solubility parameters
described in detail in Marinaccio et al.
The casting solution for forming the preferred nylon micro-
porous membrane is a nylon polymer in a solvent system for the
polyrner. The solvents which can be used with alcohol soluble nylons
include solvents such as lower alkanols, e.g. methanol, ethanol and
butanol, and mixtures thereof. It is known that nonalcohol solubie
nylons will dissolve in solvents of acids~ for example, formic acid,
citric acid, acetic acid, maleic acid and similar acids. The nylon
&@~
~z~
-26-
solutions after formation are diluted with a nonsolvent for the nylon
which is miscible with the nylon soiution. Dilution with non solvent
may9 according to Marinaccio et a!, be effected up to the point of
incipient precipitation of the nylon. The nonsolvents are selected on
the basis of the nylon solvent utilized. For example, when water
miscible nylon solvents are employed, water cqn be the nonsolvent.
General Iy, the nonsolvent can be water; methyl formate; aqueous
lower alcohols, such as methanol and ethanol; polyols such as
glycerol, glycols, polyglycols9 and ethers and esters thereof; and
mixtures of the aforementioned.
The aforementioned P application describes another similar
method which may be used for the conversion of nylon polymer into
nylon microporous mernbrane. Broadly, Pall provides a process for
preparing skinless hydrophilic alcohol-insoluble polyamide resin frorn
a polyarnide casting solution. The casting solution is formed by
inducing nucleation of the solution by the controlled addition of a
nonsolvent for the polyamide resin to obtain a visible precipitate of
polyamide resin particles.
The casting solution, e.g. whether that of Marina cio et al or
Pall, is then spread on a substrate, i.e. reinforcing web or non-porous
substr~te, to form a thin film thereon. The cqst film is then
contacted with a quenching bath comprising a non-solvent system for
the polymer for a time sufficient to form micropores in the film.
The preferred quench bath for Forming Q nylon microporous mem-
2~76
-27 -
brane comprises a nonsolvent system of methanol and water or
formic acid and water.
These preferred nylon membranes, i.e. described in Marinaccio
et al and Pall, are characterized by an isotropic structure, having a
high effective surface area and a fine internal microstructure of
controlled pore dimensions with narrow pore size distribution and
adequate pore volume. For example, a representative 0.22 micron
rated nylon 66 membrane (polyhexamethylene adipamide) exhibits an
Initial Bubble Point (113P) of about 45 to 50 psid~ a Foam All Over
Point (FAOP) of about 50 to 55 psid, provides a flow of from 70 to 80
ml/min of water at 5 psid (47 mm. diameter discs), has a surface area
(BET, nitrogen adsorption) of about 13 m2/g and a thickness of about
4.5 to 4.75 mils.
In general, the microporous filter membrane will be cast at
thicknesses in the range of from about I mil to about 2û mils,
preferably from about i to about 10 mils (wet thickness). After the
polymer solution is cast and quenched, the membrane is removed
from the quench bath and substrate upon which it was cast and then
washed.
The washed membrane is then, preferably, laminated to another
washed membrane, or optionally laminated to a web by methods well
known in the art, to form a reinforced laminated filtration mem-
brane~ A unique reinforced membrane is described and claimed in
Canadian Serial No. 417,737 to Barnes et al filed
~ecember 15, 1982. Preferably, laminatlon is accomplished
-28-
by passing the plurality of layers juxtaposed upon each
other through heated rollers to heat laminate and dry the
membranes together. Preferably such drying is under
restraint to prevent shrinkage. Drying of the membranes
under restraint is described in U.S. Defensive Publication
T103,601 published ~ovember 1, 1983. Generally, any
suitable restraining technique may be used while drying,
such as winding the membrane tightly about a dry surface,
e.g. a drum. Biaxial control is preferred and tensioning
the laminated membrane is considered the most preferred.
The final drying and curing temperature for the
filtration membrane should be sufficient to dry and cure the
the membranes. Preferably this temperature is from about
120C to 140C for minimization of drying time without
embrittlement or other detrimental effects to the membranes.
The total thickness of the filtration membrane is preferably
from about 3 mils to about 30 mils and most preferably about
5 to 15 mils thick (dry thickness).
The filtration membrane may then be rolled and stored
under ambient conditions for further processing. After
formation of the membrane, it may be treated in accordance
with U.S. Patent ~o. 4,473,474 to Ostreicher et al, to
produce a cationically charged modified microporous Membrane
particularly suitable for the filtration of parenteral or
biological liquids, or in accordance with Canadian Serial
~Z~ 7~;
-29
No~ 404,133 filed May 31, 1982 to Barnes et al, now Canadian
Patent No. 1,166,411, to produce another type cationically
charged modified microporous membrane, particularly suitable
for the filtration of high purity water, i.e. at least 18
megohm-cm resistivity, required in the manufacture of
electronic component.
A method of producing an integrally preformed non-porous
sealing area of the filter membrane is by collapsing the
pores of the membrane to produce the sealing area. In order
to produce the preferred form of the filter membrane, which
comprises an elongated porous filtration area longitudinally
bordered by substantially nonporous sealing areas, by this
method, the apparatus of Fig. 4 may be utilized.
The apparatus broadly comprises a supporting roller (44)
over which the microporous membrane (46) passes. The membrane
may be produced by any of the methods well known in the art,
preferably by the aforementioned Marinaccio_et al process.
Crushing rollers (48), each of which has a force (F)
applied thereto, are then ro]led along the longitudinal
borders of the filtration area, collapsing the pores of
the membrane against the supporting drum to produce the
non-porous areas (50). Such a method of producing the mem-
brane partially and/or totally collapses the pore structure
of the membrane in the area between the supporting roller
i
o~
~30-
(44) and crushing roiler (48). This collapsed pore structure is both
strong and flexible. It is possible to collapse nylon membrane to a
smooth transparent film at high crushing forces (F), however, this
degree of collapse is not necessary to accomplish the objects of this
invention. Bubble poînt tests reveal that the crushed area has a
bubble point too high to measure by conventional means, i.e~ the area
is substantially non-porous. Additionally, the first bubble does not
appear at the interface but well within the filtration area indicating
that there is no pore damage at the interface.
Referring to Fig. 4A9 it may be desirable to provide the
crushing roller (48) with a taper at the surface which is near the
interface between the porous (46~ and non-porous areas (50~ to
prevent a steep fault Iine between these areas. Optionally, the area
which is to be crushed may be cast thicker to provide a filter
membrone of constant thickness after crushing. Referring to Fig. 4B,
it mqy also be desirable to provide the roller (48) with a cutting
surface (52) on the end to simultaneously trim and crush the
membrane.
Another method of producing the preferred elongated micro-
porous filter membrane having an elongated porous filtration area
longitudinal Iy bordered by an integral Iy substantial Iy non-porous
sealing area, is by a sirnultaneous casting and quenching method.
General Iy, this involves the known methods of producing the
membrane by the aforementioned l\Aarinaccio et al and Pal I
references. Referring to Fig.5, which depicts an apparatus in
.
',
~o~
schematic which may be utilized to prepare the membrane of this
invention, a first (54) and second (56) casting solution are prepared.
Each casting solution has an amount of film forming polymer in a
solvent system. Both of these casting solutions are simultaneously
cast through casting boxes ~58, 6û,62) on to a casting surface (64) to
provide respectively, an elongGted cast area (66) borciered by two
edge cast areas (68, 7û)9 each edge cast area being adjacent to and in
contact with longitudinal edges of the elongated cast area. These
cast areas are then simultaneously contacted with a quench bath (72),
for a time sufficient to form the appropriate micropors in the cast
areas. The amount of polymer, the solvent system and non-soivent
system are each individually selected to provide the edge areas with
a pore size less than the filtration area. Preferably these edge areas
are substantially non-porous. As indicated previously, the preferred
film forming polymer is nylon.
Preferqbly, the solvent systems comprise a mixture of at least
one soivent and one non-solvent for the polymer. The amount of non-
solvent used is preferably not greater than an amount required to
induce neucleation of the soiution to obtain a visible precipitate, the
solvent being formic acid and the non-solvent being selected frorn the
group consisting of rnethanol, methylformate, water and glycerols.
The non-solvent system is preferably a mixture of methanol and
water or formic acid and water~
The manner of varying the amount of polymer, solvent systems
and non-solvent systems to qchieve differing pore size in a
~QI~
-32-
membrane, particuiarly a nylnn membrane, are well known in the art
from, for example, the aforementioned Marinaccio et al and Pall.
Generally, by increasing the ratio of non-solvent to polymer in the
casting solution the pore size of the membrane produced wili
decrease. For example, a cqsting solution containing 16% by weight
nylon, a methanol/nylon ratio of .445 and the balance formic acid
when cast into a quench bath containing 30% methanol3 2.0% formic
acid and 68% water by weight produces a membrane having about .2
micron pores. Increasing the meth~nol/nylon ratio to .48, with all
other parameters remaining the same, produces c membrane with
reduced pore size (See ExGmple IV).
Additionally, the selection of the qmount of polymer, solvent
and non-solvent systems should be selected so that the character-
istics of the interface between the membranes is not deletoriously
effected by cross diffusion between the two casting solutions.
A preferred process for making the preferred filter element of
this invention (which is included within the generic concept of this
invention) is described and claimed in concurrently
~iled Canadian Serial No. 408,403, filed on July 29,
1982 by Mi.ller. This preferred filter element
comprises a hydrophilic nylon microporous filter
membrane hqving a preformed substantially non-porous sealing area
of non-porous tape which is heat sealed to the membrane~ The filter
housing has a hydrophobic thermoplastic sealing surface in thermo-
plastic sealing relationship with the sealing area. Preferabiy, as
:, .
-33-
described in detail in this concurrentiy filed applioation, the tape is
comprised of q layer of polyester coated with heat sealable poly-
ethylene. The apparatus used for applying the tqpe to the preferred
form of the filter membrane is also described in detail in this
concurrently filed applicqtion. Generaliy, the preferred process for
making this filter element comprises preparing a filter membrane
having an elongated porous filtration area by methods well known in
the art, and then applying a heat sealable non-porous tape to the
longitudinal borders of the filtration area.
This preferred embodiment of the filter element of this
invention has the advantages in that the nylon membrane can be
utilized with the preferred filter housing~ i.e. polypropylene, is simple
to manufacture and there are no solvents employed to adhere the
tape to the membrane or the filter housing to the membrane. The
tape also adds to the structural rigidity of the rnembrane permitting,
for example, lower temperatures for softening the end caps providing
a savings in energy and decreased injury to the membrane.
For so called sterile filtrations, involving biological liquids, the
filter element is santiized or sterilized by autoclaving or hot water
flushing prior to use. The filtration element and membrane of this
invention are resistant to this type treatment, and retain their
integrity under such conditions.
Having now generally described this invention, the invention
will become better understood by reference to specific Examples.
These Examples are included herein for the purposes of illustration
~ ., -
3L~Q(~7
-34-
only and are not intended to be limitin~ of the invention unless so
specified.
By the use of the term "preformed" it is meant that the
substantially non-porous area is formed on air in the flat membrane.
By the use of the term "integrally preformed" it is meant that
the substantially non-porous area is monolithic with the porous
filtration area qnd consist essentially olF the same organic polymeric
material as the porous filtration areo.
-35-
EXAMPLES
The following 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) diarneter platen dial indicator thickness gauge. Gauge accuracy
was + 0.00û05 inches ( +.05 mils).
Initial Bubble Point (IBP) and Foam-AII-Over
Point (FAOP) Tes~s
A 47 mm diameter disc of the membrane sarnple is placed in a
special test holder which seals the edge of the disc. Above the
membrane and directly in contact with its upper face, is a perforated
stainless steel support screen which prevents the membrane from
deforming or rupturing when air pressure is applied to its bottom
face. Above the membrane cmd support screen, the holder is provided
with an inch deep cavity into which distilled water is introduced. A
regulated air pressure is increased until a first stream of air bubbles
is emitted by the water wetted membrane into the quiescent pool of
water. The air pressure at which this first stream of air bubbles is
emitted is called the Initial 8ubble Point (IBP) - see ASTM F316 7.
Once the Initial Bubble Point pressure has been determined and
recorded, the air pressure is further increased until the air flow thru
the wetted rnembrane sample, as measured by a flow meter in the
line between the regulator and the sample holder, reaches lû0 cc/min.
The air pressure at this flow rate, is called Foam-AII-Over-Point
(FAOP).
~o~
--3 -- .
Flow Rate Test
A 47 mm diameter disc of fhe membrane sample is placed in a
testing hovsing which allows pressurized water to flow thru the
membrane. Prefiltered water is passed thru the membrane sample at
a pressure differentiql of 5 psid. A graduated cylinder is used to
measure the volume of water passed by the membrane sample in a
one rninute periodO
(3'7~ Ei
-37-
EXAMPLE I
PREPARATION OF A,~ICROPOROUS MEMBRANE
~0.2 MICR()NS)
A representative nylon 66 membrane having a nominal surface
area of about 13 m2/g and a nominal pore size ratina o-f û.2 rnicrons
was prepared by the method of Marinaccio et al, U.S. Patent No.
3,876,738, utilizing:
I) a casting solution of approximately 16 percent by
weight nylon 66 (Monsanto Vydyne 66B), and the proper mixture
of solvent formic acid and non-solvent methanol (see Tnble 1) to
produce the desired minimum Initial Bubble Point of 42 psi, at
the necessory dope mixing temperature, time, pressure, etc. (as
required by the method of Marinaccio et al);
2) a quench bath composition of 3û% methanol, 2.0%
formic acid and 68% water by weight (regenerated as required
by the method of Knight et al, U.S. Patent No. 3,928,517~;
3) a casting speed of 96 inches/min on a 30" diameter
casting drum; and
4) a quench bqth temperature of 23C.
The membrane was produced by casting the casting solution
onto the drum just under the surface of the quench bath approxi-
mately 4.5 mils thick qs cast wet, to obtuin a dry single layer of
approximately 2.5 mils thick. The membrane was allowed to separate
- ,
-38-
from the drum at about i800 of qrc from the point of application and
was guided out of the quench bath and into a series of high purity
water rinsing zonesO The wet membrane was then slit from the cast
40" width down to two 15'1 widths, anci taken up on separate hollow
cores in lengths of 2ûû feet. These outer membrane layers may be
stored wet in this fashion for several days before subsequent
processing. The Membrane Chnracteristics for these layers are
obtained after drying a double-iayered sample of this membrane
under restraint conditions .
~.
, .
~2~t7~
-39-
TABLE I
~stin~ Solution Composition
Nylon 16.06 wt. %
Formic Acid 77.01 wt. %
Methanol 6.93 wt. %
Castin~ lution Mixi~ Condi7ions
Temperature 30C
Time 5 hrs.
Pressure psig 1.0 psig
~embrcme Ch~racteristics
. . _. . _
IBP FAOP THICKNESS Q
(~si) (psi~ ~mils) (cc/min.)
42.3 5 1 .5 5.38 81.3
.
.
3~37~6;
-40-
EXAMPLE 11
PREPAP~ATION F MICROPOROUS MEMBRANES
45 MICRONS)
A representative nylon 66 membrane having a nominal surface
pore size rating of 0.45 microns was prepared by the same method
described in Example 1. Changes in the casting solution composition
and mixing conditions are noted in Table ll.
-41 -
TABLE 11
Casting Solution Composition
Nylon 16.15 wt. %
Formic Acid 77.45wt. %
Methanol 6.40 wt. %
CQstin~ ~1
Temperaturc 300C
Time 5.0 hrs.
Pressure psig l.û psig
Membrcme Ch~rclcteristics
IBP FAOP THICKNESS Q
(~ (mi Is) (cc/min
29.7 34 5.83 1 70
~ V7 6 ~
-42 -
EXAMPLE 111
PREPARATION OF MICROPOROUS MEMBRANES
(0.65 MICRONS)
A respresentative nylon 66 membrane having a nominal pore
size rating of 0.65 microns was prepared by the sqme method
described in Example 1, except that the mernbrane was produced at a
casting surface speed of 30 inches/min. on a casting drum of 12.5
inch diameter. The we~ thickness of a sheet was 5.5 mils. Changes
in the casting solution composition and mixing conditions are noted in
the Table 111.
:
.
3(~ 6:
-43 -
TABLE 111
Casti~ ti~n Cu~
Nylon 1 6.00wt. %
Formic Acid 78.04wt~ %
Methanol 5.96 wt. %
Casting Soiution M xin~ Conditions
Temperature 300C
Time 3 hrs~
Pressure psig 0.0 psig
Menbr~ Cl~
IBP FAOP THICKNESS Q
~2 ~ (mils) ~cc/min~)
1 6.0 1 9.5 3.90 620
-44-
SIMULTANEOUS C:ASTIN(;
AND QUENCHINC;
Samples of the preferred membrane of this invention were
prepared on a device similar to that shown in Fig. 5. The casting
solutions used were as foliows:
Conventional 0.2 micron "Non-Porous" Edge
Membrane Castinq Solution
Nylon -16 wt. % Nylon ~ 16 wt. %
Methnol/Nylon = o.445 Methanol/Nylon = 0.48
Formic Acid = balance Formic Acid = balance
Quench Bath - see Example I
The membrane was cast so that half of the sheet width was cast
from the conventional casting solution while the other half was cast
from the modified "Non-porous'~ casting solution. The separator in
the casting box was relieved so that the two soiutions could contact
each other just as they moved under the casting knife into the ,quench
b~th.
The r esulting membrane sheet exhibited a sharp visual
demarcation between the halves. The half cast frorn the
Conventional Casting Solution was white and opaque. The half cast
from the Non-Porous Casting Solution was translucent. There were
no apparent mechanical discontinuities between the two adjacent
halves except the translucenf half was slightly (û.2 to û.3 mils)
' ~
12Qf~
-~5 -
thinner. Bubble point tests were conducted on hoop dried samples,
with results as follows:
White, oeaque ''Conventional Castin~ Solution half:
IBP=49psi
FAOP = 54 psi.
Translucent "modifie~porous" half: no indication of any
bubbles up to 90 psi (limit of test apparatus~
Interface between halves: IBP = 54 psi.
These results indicate that the desired membrane sheet
structure was obtained; namely, a conventional porous 0.2 um mem-
brane (IBP 7~ 45) with a non-porous edae (IBP 90 psi), with no
structural defects or bubble point depression at the interface,
utilizing a simultaneous casting and quenching technique.
The use of such a non-porous edged membrane in the fabrica-
tion of pleated filter cartridges should eliminate all of the problems
of bubble point depression that occur at the interface betwen the
membrane ~nd the normally hydrophobic end cap and/or hydrophobic
adhesive.
.
-~6-
EXAMPLE V
COLLAPSING PORES
A dry doubie layer of nylon rnembrane was prepared pursuant to
ExGmple 1. A sample of the membrane was crushed between a glass
plate and a stainless steel rol ler bearing by moving the smooth
surfaced roller across the stationary membrane laid upon the glass
plate. By varying the pressure of the roller bearing on the mem-
brane9 different degrees of crushing were obtained. A high pressure
turns the white, opaque membrane to a clear transparent state.
The crushed area had an IBP of over 90 psi. The interface
between the crushed and non-crushed area was bubble point tested
(IBP, FAOP). The non-crushed membrane area had an IE~P of 42 psi.
The first bubble did not occur at the interface between the crushed
and non-crushed areas but well within the non-crushed area. The
original membrane prior to crvshing had an IBP of 43.8 psi, avg.
thickness of 5.07 mils~ FAOP of 47.7 psi, Flow Rate Avg. of 87.7
ml/min. @ 5 psid in a 47 mm sample holder.
.. . .. .
