U.S. patent application number 11/254962 was filed with the patent office on 2006-02-16 for pre-metered, unsupported multilayer microporous membrane.
Invention is credited to Eugene Ostreicher, Richard Sale.
Application Number | 20060032812 11/254962 |
Document ID | / |
Family ID | 32830725 |
Filed Date | 2006-02-16 |
United States Patent
Application |
20060032812 |
Kind Code |
A1 |
Sale; Richard ; et
al. |
February 16, 2006 |
Pre-metered, unsupported multilayer microporous membrane
Abstract
An at least two layer, unsupported, continuous microporous
membrane is disclosed. The at least two layer, unsupported,
continuous microporous membrane may include at least two different
membrane pore size layers or the pore sizes may have about the same
pore size. Apparatus and processes for fabricating at least a two
layer unsupported, continuous, microporous membrane are also
disclosed. One representative process disclosed for forming a
continuous, unsupported, multilayer phase inversion microporous
membrane having at least two layers comprises of the acts of:
operatively positioning at least one dope applying apparatus,
having at least two polymer dope feed slots, relative to a
continuous moving coating surface; applying polymer dopes from each
of the dope feed slots onto the continuously moving coating surface
so as to create a multiple layer polymer dope coating on the
coating surface; subjecting the multiple dope layer to contact with
a phase inversion producing environment so as to form a wet
multilayer phase inversion microporous membrane; and then washing
and drying the membrane. Other representative apparatuses and
processes are also disclosed.
Inventors: |
Sale; Richard; (Tolland,
CT) ; Ostreicher; Eugene; (Farmington, CT) |
Correspondence
Address: |
CUNO INCORPORATED
400 RESEARCH PARKWAY
P. O. BOX 1018
MERIDEN
CT
06450-1018
US
|
Family ID: |
32830725 |
Appl. No.: |
11/254962 |
Filed: |
October 20, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10771801 |
Feb 4, 2004 |
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11254962 |
Oct 20, 2005 |
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10072202 |
Feb 7, 2002 |
6736971 |
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11254962 |
Oct 20, 2005 |
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09923640 |
Aug 7, 2001 |
6706184 |
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11254962 |
Oct 20, 2005 |
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60223359 |
Aug 7, 2000 |
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Current U.S.
Class: |
210/490 ;
210/500.27; 210/500.42 |
Current CPC
Class: |
B01D 2323/08 20130101;
B01D 71/56 20130101; B05C 5/0254 20130101; B01D 71/34 20130101;
B32B 27/08 20130101; B01D 2323/42 20130101; B32B 2255/26 20130101;
B01D 67/0013 20130101; B01D 67/0018 20130101; B01D 69/02 20130101;
B01D 67/0011 20130101; B29C 41/32 20130101; B01D 2325/022 20130101;
B32B 5/32 20130101; B32B 27/304 20130101; B01D 69/06 20130101; B32B
2305/026 20130101; B01D 69/12 20130101; B01D 71/68 20130101; B05B
9/06 20130101 |
Class at
Publication: |
210/490 ;
210/500.27; 210/500.42 |
International
Class: |
B01D 71/06 20060101
B01D071/06 |
Claims
1. A multilayer, unsupported, microporous membrane comprising: a
first layer and at least a second layer having a distinct change in
pore size between each of the layers, the first and the at least
second layers being connected at the interface such that the
multilayer microporous membrane is continuous.
2. The multilayer membrane of claim 1 wherein the first layer is
formed from a first polymer dope for producing one pore size and
the at least a second layer is formed from at least a second
polymer dope for producing at least one different pore size.
3. The multilayer membrane of claim 1 wherein the polymer dope
comprises: nylon.
4. The multilayer membrane of claim 1 wherein the polymer dope
comprises: polyvinylidene fluoride.
5. The multilayer membrane of claim 1 wherein the polymer dope
comprises: polyether sulfone.
6. The multilayer membrane of claim 1 wherein the multilayer
membrane has a type I configuration.
7. The multilayer membrane of claim 1 wherein the multilayer
membrane has a type III configuration.
8. The multilayer membrane of claim 1 wherein the multilayer
membrane has a type IV configuration.
9. The multilayer membrane of claim 1 wherein the multilayer
membrane has a type V configuration.
10. The multilayer membrane of claim 1 wherein the multilayer
membrane has a type VI configuration.
11. The multilayer membrane of claim 1 wherein the multilayer
membrane has a type VII configuration.
12. The multilayer membrane of claim 1 wherein the multilayer
membrane has a type VIII configuration.
13. The multilayer membrane of claim 1 wherein the multilayer
membrane has a type IX configuration.
14. A two layer, unsupported, microporous membrane comprising: a
first layer having a first distinct pore size; and a second layer
having a distinct change in pore size between each of the layers,
the first and the second layers being connected at the interface
such that the two layer multilayer microporous membrane is
continuous.
15. A multilayer, unsupported, microporous membrane comprising: a
first layer having a first distinct pore size; and at least a
second layer having at least a second distinct pore size, each of
the layers being connected at the interface therebetween such that
the multilayer microporous membrane is continuous and has a
distinct change in pore size at each interface therebetween.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of commonly owned U.S.
patent application Ser. No. 10/771,801, filed Feb. 4, 2004 of Sale
et al., entitled "Pre-metered, Unsupported Multilayer Microporous
Membrane," a continuation-in-part of commonly owned U.S. patent
application Ser. No. 10/072,202, filed Feb. 7, 2002 of Sale et al.,
entitled "Pre-metered, Unsupported Multilayer Microporous
Membrane," U.S. patent application Ser. No. 09/923,640, filed Aug.
7, 2001 of Sale et al., entitled "Unsupported Multizone Microporous
Membrane," which is a continuation-in-part of commonly owned U.S.
Provisional Patent Application Ser. No. 60/223,359, filed Aug. 7,
2000, of Sale et al., entitled "Unsupported Multizone Microporous
Membrane," the disclosure of each is herein incorporated by
reference to the extent not inconsistent with the present
disclosure.
BACKGROUND OF THE DISCLOSURE
[0002] The present disclosure relates to continuous, unsupported,
microporous membranes having two or more distinct, but controlled
pore sizes and to processes of making and using same, more
particularly to unsupported microporous membranes made from a first
dope and at least one additional dope being applied directly to one
another prior to the at least two dopes being quenched and to
apparatus for manufacturing and processes for making such
membrane.
[0003] Microporous phase inversion membranes are well known in the
art. Microporous phase inversion membranes are porous solids which
contain microporous interconnecting passages that extend from one
surface to the other. These passages provide tortuous tunnels or
paths through which the liquid which is being filtered must pass.
The particles contained in the liquid passing through a microporous
phase inversion membrane become trapped on or in the membrane
structure effecting filtration. The particles in the liquid that
are larger than the pores are either prevented from entering the
membrane or are trapped within the membrane pores and some
particles that are smaller than the pores are also trapped or
absorbed into the membrane pore structure within the pore tortuous
path. The liquid and some particles smaller than the pores of the
membrane pass through. Microporous phase inversion membranes have
the ability to retain particles in the size range of from about
0.01 or smaller to about 10.0 microns or larger.
[0004] Many important micron and submicron size particles can be
separated using microporous membranes. For example, red blood cells
are about eight (8) microns in diameter, platelets are about two
(2) microns in diameter and bacteria and yeast are about 0.5
microns or smaller in diameter. It is possible to remove bacteria
from water by passing the water through a microporous membrane
having a pore size smaller than the bacteria. Similarly, a
microporous membrane can remove invisible suspended particles from
water used in the manufacture of integrated circuits in the
electronics industry.
[0005] Microporous membranes are characterized by bubble point
tests, which involve measuring the pressure to force either the
first air bubble out of a fully wetted phase inversion membrane
(the initial Bubble Point, or "IBP"), and the higher pressure which
forces air out of the majority of pores all over the phase
inversion membrane (foam-all-over-point or "FAOP"). The procedures
for conducting initial bubble point and FAOP tests are discussed in
U.S. Pat. No. 4,645,602 issued Feb. 24, 1987, the disclosure of
which is herein incorporated by reference to the extent not
inconsistent with the present disclosure. The procedure for the
initial bubble point test and the more common Mean Flow Pore tests
are explained in detail, for example, in ASTM F316-70 and ANS/ASTM
F316-70 (Reapproved 1976) which are incorporated herein by
reference to the extent not inconsistent with the present
disclosure. The bubble point values for microporous phase inversion
membranes are generally in the range of about two (2) to about one
hundred (100) psig, depending on the pore size and the wetting
fluid.
[0006] An additional method which describes a pore measurement
technique is ASTM E1294 89 which describes a method for determining
pore size by clearing fluid from the pores of the membrane and
measuring the resulting flow. This method is used to measure mean
flow pore but is similar to the method of Forward Flow Bubble Point
in that the wet portion of the test uses a similar protocol.
[0007] The Forward Flow Bubble Point (FFBP) test is described in
U.S. Pat. No. 4,341,480 by Pall et. al., the disclosure of which is
herein incorporated by reference to the extent not inconsistent
with the present disclosure. This patent discloses how the FFBP can
be used to distinguish a symmetric membrane from an asymmetric
membrane. The FFBP curve is generated by saturating the membrane
with fluid and subjecting one side to a rising air pressure while
measuring air flow on the downstream side. For a single layer
symmetric membrane with a well defined pore size, a plot of air
flow versus pressure remains flat but slightly above zero due to
diffusion through the liquid in the membrane. When the pressure
reaches a point when it can overcome the surface tension of the
fluid in the pore, the air will push the liquid out of the pore and
air will flow through the pore in bulk (bulk flow). This pressure
point is a function of the surface tension of the liquid and the
radius of the pore as defined by the equation of Young and Laplace
(see Physical Chemistry of Surfaces by Arthur Adamson, Wiley
Press), When the pores all have essentially the same size, this
event occurs simultaneously and is characterized by a transition of
the flow versus pressure curve from horizontal (when diffusion flow
is predominant) to vertical (where bulk flow is dominant), this
type of FFBP characteristic is shown in FIG. 9. FIG. 9 also
demonstrates that the FFBP characteristics for a single layer
symmetric membrane are identical regardless of membrane
orientation.
[0008] On the other hand, asymmetric membranes are characterized by
a gradual change in pore size throughout the thickness and exhibits
a different FFBP curve. When tested with the large pore size
surface facing up stream against the applied air pressure. Since
the pore size is gradually changing throughout the thickness depth,
the pressure required to push fluid down the pores rises gradually
and the resulting FFBP curve has a rising slope until the final
bubble point is reached and bulk flow occurs. While an asymmetric
membrane might be retentive, the above response is
indistinguishable from an asymmetric membrane with defects, where
certain pores are significantly larger than the remaining pores and
exhibit bulk flow at lower pressure. The FFBP response of this type
of membrane also exhibits a rising slope when flow versus pressure
is plotted.
[0009] U.S. Pat. No. 3,876,738, the disclosure of which is herein
incorporated by reference to the extent not inconsistent with the
present disclosure, describes a process for preparing microporous
membranes by quenching a solution of a film-forming polymer in a
non-solvent system for the polymer. U.S. Pat. No. 4,340,479, the
disclosure of which is herein incorporated by reference to the
extent not inconsistent with the present disclosure, generally
describes the preparation of skinless microporous polyamide
membranes by casting a polyamide resin solution onto a substrate
and quenching the resulting thin film of polyamide.
[0010] There is an extensive body of knowledge concerning the
production of multiple layer films using pre-metered coating
technology, such as, for example, slot dies, as taught by. This
prior art deals with the extrusion of films that are essentially
impermeable. This prior art also discusses manufacture of both
photographic film and films used in the packaging industry (e.g.
food packaging). Some examples of patents, each of which are herein
incorporated by reference to the extent not inconsistent with the
present disclosure, disclosing multilayer films are listed in the
table below: TABLE-US-00001 Patent Issued Inventor(s) Title U.S.
Pat. No. 6040392 2000 Khanna et. al. Nylon 6 or 66 Based
Compositions and Films Formed Therefrom Having Reduced Curl. U.S.
Pat. No. 5962075 1999 Sartor et. al. Method of Multilayer Die
Coating Using Viscosity Adjustment U.S. Pat. No. 5741549 1998 Maier
et. al. Slide Die Coating Method and Apparatus with Improved Die
Tip U.S. Pat. No. 5256357 1993 Hayward Apparatus and Method for
Cocasting Film Zones U.S. Pat. No. 4854262 1989 Chino et. al.
Coating Apparatus U.S. Pat. No. 4001024 1977 Dittman et. al. Method
of Multilayer Coating
[0011] At least some of the above prior art teaches the use of
pre-metered dies to apply coatings in the production of essentially
non-porous films. Discussion of pre-metered dies can be found in
two Troller Schwiezer Engineering (TSE) publications, "Concepts and
Criteria for Die Design" and "Precision Coating: Pre-metered and
Simultaneous Multilayer Technologies," which are available from TSE
upon request. Pre-metered coating methods comprise slot, extrusion,
slide and curtain coating. Pre-metered coating processes are
characterized by the fact that the down-web thickness of the coated
layer is solely determined by the ratio of volumetric flow
rate/width of fluid pumped into the die to the speed of the web. A
discussion of multiple slot dies are also presented in a Master
Thesis written by Shawn David Taylor titled Two-Layer Slot Coating:
Study Of Die Geometry And Interfacial Region at McMaster University
dated July 1997, the disclosure of which is hereby incorporated by
reference to the extent not inconsistent with the present
disclosure.
[0012] Other art involves the manufacture of microporous membranes
by other techniques. Grandine provides the first practical
disclosure of the manufacture of PVDF membrane. The Grandine patent
(U.S. Pat. No. 4,203,847) discloses, although does not claim, that
thermal manipulation of the dope will lead to a change in pore size
of the resulting membrane. Surprisingly, given that nylon is a very
different polymer that is dissolved in ionic organic acids rather
than an organic ketone, it experiences a similar phenomenon.
Grandine did not suggest a mechanism for this phenomenon to
indicate that it might be general for polymers used to make
membranes.
[0013] Subsequent patents relating to PVDF disclose methods for
making asymmetric PVDF membrane. The Wang patent (U.S. Pat. No.
5,834,107) discloses a variety of methods to manufacture asymmetric
membrane. Other patents that are related to asymmetric structure
and which are cited in the Wang patent are Costar (WO 93/22034),
Sasaki (U.S. Pat. No. 4,933,081), Wrasidlo (U.S. Pat. No. 4,629,563
& U.S. Pat. No. 4,774,039), and Zepf (U.S. Pat. Nos. 5,188,734
& 5,171,445).
[0014] Asymmetric membrane prior art does not disclose, suggest or
teach independent control of the properties of each zone (such as
thickness or pore size) nor the formation of distinct layers or two
distinct polymer dopes.
[0015] Other prior art is the use of thermal manipulation to create
distinct zones of controlled pore size with nylon membrane by
Meyering et. al. as disclosed in (PCT publication WO 99/47246, the
disclosure of which is incorporated herein by reference to the
extent not inconsistent with the present disclosure) applying two
layers of dope against opposite sides of a support scrim after the
scrim was filled with a first dope. In some applications,
especially pleated cartridge filters, Nylon is an intrinsically
weak material which requires the use of a scrim to function in
particular applications effectively, but unreinforced or
unsupported nylon is used in other applications. The presence of
the reinforcing or supporting scrim requires multiple dies, one to
provide dope within and to fill the scrim for the middle membrane
zone and the other two dies to apply the dope for the outer two
membrane zones. In order for the Meyering process to effectively
operate, a centrally positioned porous support which remains with
the finished membrane is clearly required regardless of whether or
not the membrane polymer requires the support for a particular end
use application.
[0016] Additional prior art is Degen (U.S. Pat. No. 5,500,167)
which also claims a supported membrane with a porous nonwoven
fibrous support wherein the two zones of the membrane are divided
into zones of different pore sizes. In that case, a second dope
layer to form a second zone is applied to a first dope layer in a
secondary, sequential operation with the scrim partially outboard
of the two finished zones.
[0017] Additional prior art is Holzki U.S. Pat. No. 5,620,790 which
describes a multilayer membrane applied with a doctor blade but is
subject to the restriction that the viscosity of the first layer in
the polymer solution form must be greater or equal to the viscosity
of subsequent layers. This viscosity restriction require either
solids manipulations or the addition of viscosity enhancing agents
to control membrane formation. Adulteration of the polymer solution
in this manner is less desirable than a technique which is not
sensitive to viscosity differences between the layers.
[0018] Tkacik U.S. Pat. No. 5,228,994 mentions in passing, although
does not claim, that membranes may be coextruded in a multilayer
sequence examples only reference methods that are suitable for
coating a polymer solution layer to a already formed substrate,
which is the main topic of the patent. This patent does not
disclose a method to produce a multilayer membrane where neither
layer has previously been subjected to phase inversion.
[0019] Steadly U.S. Pat. No. 4,770,777 deals with skinned
multi-layer membranes made by a post-metering process.
[0020] PCT publication WO 01/89673 A2 to Kools, the disclosure of
which is herein incorporated by reference to the extent not
inconsistent with the present disclosure, appears to disclose
`co-casting` as a method to make multilayer PVDF membrane. Having
as its salient point post-metering coating apparatus which
apparently results in high interfacial shear turbulence which
results in an asymmetric transition zone, having a different pore
size from either of the two adjacent layers, in the interface zone.
It is believed that the Kools structure, as disclosed will result
in the undesirable FFBP as discussed below.
[0021] All of these preceding methods disclose the use
post-metering processes, which utilize apparatus, such as, for
example, casting knives or doctor blades to produce membranes,
whether they are single or multi-layer. The casting knife is a
post-metered approach where the thickness of the applied polymer
solution is controlled by the application of a device, such as, for
example, a spreading bar in contact with the top surface layer of
the coated material as it is applied to the substrate. These
methods suffer from the limitation that they create an asymmetric
region at the interface presently believed due to the shear action
of post-metered coating apparatus.
[0022] Another approach to joining two different membrane zones
together so as to produce a multilayer membrane is wet laminating
wherein membrane precursors that have been cast and quenched but
not dried are joined under mild pressure and then dried together.
When the pore sizes are different from each other and both layers
are symmetric, the asymmetric transition is eliminated and a
desirable FFBP curve response is generated, as shown in FIG. 8.
However, w et lamination is prone to delamination, which can be a
particular concern if the membrane is back-flushed. As a practical
matter, laminated multilayer membranes tend to be thicker than
single zone membranes since each zone is an independently,
individually prepared membrane which included being quenched prior
to being laminated together to form the multilayer membranes. These
prior art membranes are clearly relatively thick, as each zone of
the laminated multilayer membrane must be individually sufficiently
thick in order to survive the membrane manufacturing process and
then be joined with at least one other individual sufficiently
thick membrane, individually and separately prepared, to form a
multilayer laminated membrane.
[0023] Prior art on pre-metered application technology, which
includes the use of slot dies, prior art on generally does not deal
with and is not believed to have been applied to the manufacture of
microporous membranes nor its requirements for the manufacture of
microporous membranes with the exception of the Meyering et al.
disclosure mentioned above.
[0024] Thus, there is a need for unsupported or scrimless,
multilayer polymeric microporous membrane having at least two
independent and distinct pore size layers which progressing through
the thickness of the membrane, each layer being continuously joined
to its' adjacent layer throughout the membrane structure. Such a
multilayer membrane should eliminate the need for reinforcing or
supporting scrim while realizing the advantages of multilayer
filtration control. Such a scrimless multilayer membrane should
have at least two separate layers that are continuously joined by
the molecular entanglement that occurs in the liquid between the
two dope layers prior to phase inversion but with a sharp pore size
transition between the two layers. Such a multilayer scrimless
membrane should be preferably as thin as prior art single layer
membranes and thinner than prior art laminated multilayer
membranes. Such a membrane should exhibit a FFBP curve such that it
can be distinguished from a defective membrane.
SUMMARY OF THE DISCLOSURE
[0025] The present disclosure is directed to unsupported (without
an integral reinforcing or supporting porous support) multilayer
microporous membrane, apparatuses and processes for the manufacture
thereof. The unsupported membrane may be substantially
simultaneously formed into multiple (two or more) discrete layers,
each with, presently preferably, a different but controlled pore
size. The unsupported membrane may also comprise multiple (two or
more) discrete layers each with, presently preferably, a different
but controlled pore size, with a distinct change in pore size at
the interface between each of the layers that does not exhibit
locally asymmetric pore size distributions, such that the resulting
membrane exhibits a Type I Forward Flow Bubble Point (FFBP) curve
response, as illustrated in FIGS. 16 a and 16 c, and demonstrated
in FIG. 12 and discussed below.
[0026] Layers of dope that eventually form the layers are applied
directly to one another prior to the membrane quench such that
interfacial turbulence and gross mixing between adjacent layers are
avoided, maintaining distinct pore sizes within the separate layers
but where the separate layers are integrally joined at each
interface. A multilayer membrane structure results from the process
step of applying the individual dopes or polymer solutions that
form each of the layers sequentially onto one another, the
resulting multilayer liquid coating, subsequent to being subjected
to a process step that induces phase inversion that forms the
distinctly sized pores in each layer, with each porous layer being
physically bonded to its' adjacent porous layer by polymer
intermingling, at a molecular scale, at the interface but without
any extensive intermixing in the interfacial regions between the
layers, as will be explained in more detail below.
[0027] The application is, presently preferably achieved, with a
pre-metered coating system which does not introduce any significant
shear turbulence at the interface between adjacent dope or polymer
solution layers. The present applicants have determined that this
absence of significant shear turbulence is in contrast to a
post-metered coating system, such as knife coating, which has now
been determined to create significant shear turbulence between each
of the applied liquid layers, as discussed in the Kools
publication. The applicants of the present disclosure believe that
they have replicated the Kool's post-metering process to produce a
two layer membrane. The FFBP of the Kool's membrane resembles that
shown in FIG. 7. In light of FIGS. 16 b and 16 d, the interface of
Kool's process appears to indicate the presence of a significant
asymmetric zone at the interface. The results obtained appear to
verify the disclosure as contained in the Kools publication.
[0028] This discernable transition layer occurs whether the two
dope or liquid polymer layers are applied by two separate casting
knifes located some distance apart, as in one experiment, or
whether the two casting knifes are built into a single assembly so
that there is essentially zero gap between the two polymer solution
applications, as disclosed in the Kool's publication.
[0029] The TSE documents cited previously and the Book, "Liquid
Film Coating," edited by Stephen F. Kistler and Peter M. Schweizer,
Chapman and Hall USA 1997," the disclosure of which is herein
incorporated by reference to the extent not inconsistent with the
present disclosure, lists a number of pre-metered coating
processes. These pre-metered coating processes are identified as,
but are not limited to, slot coating, extrusion coating, slide
coating, and curtain coating. While all of these process are
capable of producing multilayer polymer solution coatings without
any significant interfacial shear turbulence, it is anticipated
that for the production rates typically employed to produce
membrane and a typical viscosity of polymer solutions in the 1000
to 5000 cP range, that slot coating process will be preferred.
[0030] The concept taught could be applied to nylon, PVDF, PES, PP
or any membrane component or polymer capable of producing a phase
inversion membrane wherein pore size can be controlled or
predetermined through specific control of the polymer solution or
dope preparation, which may include formulation of constituents,
thermal manipulation, or use of any other pore size controlling
steps known to the art prior to the coating process step.
[0031] The present disclosure claims, among other innovations, the
process of coating multiple layers of polymer solutions, each
having been controlled to eventually produce a predetermined pore
size when subjected to phase inversion on to a moving nonporous
self-releasing substrate with each layer applied with a
pre-metering device such as, for example, a slot die and then
subjecting such multiple fluid layers to a phase inversion process
in, for example, a nonsolvent or solvent/nonsolvent liquid bath in
such a manner as to produce an unsupported, multilayer microporous
membrane precursor having multiple pore size layers. The moving
coating surface material, presently preferably, a nonporous
self-releasing support with each layer applied with a pre-metered
die is selected so that it is compatible with the dope or polymer
solution and will self-release from the wet microporous membrane
precursor after the phase inversion process.
[0032] It is further contemplated that the individual layers may
vary from each other in other functional aspects other than pore
size, where interlayer mixing must be avoided. Such differences
between layers may include polymer end group functionality, polymer
composition (use of copolymers), particulate fillers, additives,
different molecular weight, wetting characteristics (hydrophilic
and hydrophobic), or other functional layer differences, wherein
such differences are intrinsic to the individual dopes used to form
each of the layers and interlayer mixing must be avoided.
[0033] One aspect of the present disclosure includes a process for
forming a continuous, unsupported, multilayer phase inversion
microporous membrane having at least two distinct symmetrically
distributed pore size layers, comprising of the acts of:
operatively positioning at least one pre-metering dope applying
apparatus capable of applying at least two independently
pre-metered polymer dopes relative to a continuously moving
nonporous support coating surface; cooperatively applying the
pre-metered polymer dopes onto the continuously moving nonporous
support coating surface so as to create a multilayer polymer dope
coating on the nonporous support coating surface; and subjecting
the multilayer dope coating to contact with a phase inversion
producing environment so as to form a wet multilayer phase
inversion microporous membrane precursor, and then washing and
drying this wet precursor structure to form the desired dry
multilayer microporous membrane.
[0034] Another aspect of the present disclosure includes a process
for forming a continuous, unsupported, multilayer phase inversion
microporous membrane having at least two layers, comprising of the
acts of: operatively positioning at least two pre-metering dope
applying or coating apparatus, each capable of independently
applying at least one polymer dope, relative to a nonporous support
coating surface; sequentially applying polymer dopes from each of
the pre-metering dope applying or coating apparatus onto the
nonporous support coating surface so as to create a multilayer
polymer dope coating on the nonporous support coating surface; and
subjecting the sequentially applied polymer dopes to contact with a
phase inversion producing environment so as to form a wet
multilayer phase inversion microporous membrane precursor, washing
and drying said precursor to form the desired dry multilayer
microporous membrane.
[0035] Still another aspect of the present disclosure includes a
multilayer, unsupported, microporous membrane comprising: a first
layer having a symmetrically distributed first pore size; and at
least a second layer having a symmetrically distributed second pore
size, the first and second layers being operatively connected with
a distinct change in pore size at the interface thereof such that
the multilayer membrane is continuous and does not include any
support material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is a perspective view of a representative die useful
to produce membrane according to the present disclosure;
[0037] FIG. 2 is a schematic plan view of a representative membrane
produced according to the present disclosure;
[0038] FIG. 3 is a schematic plan view of another representative
membrane produced according to the present disclosure.
[0039] FIG. 4 illustrates a SEM cross section of an unsupported
multilayer PVDF sample 0228sd67.5 membrane cast with a doctor
blade;
[0040] FIG. 5 illustrates a SEM cross section of an unsupported
multilayer PVDF membrane sample 0410S67.5;
[0041] FIG. 6 illustrates a close-up of the interface of the two
layers of FIG. 5;
[0042] FIG. 7 illustrates two forward flow bubble point curves for
sample 0228sdr67.5 of Table 1;
[0043] FIG. 8 illustrates a forward flow bubble point curve for a
prior art laminated membrane;
[0044] FIG. 9 illustrates a single layer forward flow bubble point
curve;
[0045] FIG. 10 illustrates a cross section of a prior art laminated
PVDF membrane;
[0046] FIG. 11 illustrates a SEM close-up of the interface of the
prior art laminated PVDF membrane of FIG. 10;
[0047] FIG. 12 illustrates a forward flow bubble point curve for
nylon membrane sample 206 of table 2;
[0048] FIG. 13 illustrates a SEM cross section of membrane sample
0103 of table 2;
[0049] FIG. 14 illustrates a forward flow bubble point curve for
nylon membrane sample 0103 of table 2;
[0050] FIG. 15 is a schematic view of a representative apparatus
useful to produce membrane according to the present disclosure;
[0051] FIG. 16a is a graphic illustration of a Type I Multilayer
Membrane formed by a Pre-Metering Process
[0052] FIG. 16b is a graphic illustration of a Type II Multilayer
Membrane formed by a Pre-Metering Process;
[0053] FIG. 16c is a graphic illustration of a Type I Multilayer
Membrane FFBP test results; and
[0054] FIG. 16d is a graphic illustration of a Type II Multilayer
Membrane FFBP test results.
DETAILED DESCRIPTION OF THE DISCLOSURE
[0055] The microporous membrane and the methods for producing same
as disclosed herein can also be characterized by a FFBP curve, as
illustrated in FIGS. 16a-16d. As the result of further
consideration of the experiments included in the parent
application, we have discovered that certain multilayer membrane,
although they may appear multilayer with SEM analysis, will exhibit
a degree of asymmetry if produced by prior art methods described
below. However, as will be apparent from the present disclosure,
products and methods that produce the disclosed multilayer membrane
eliminates this asymmetry.
[0056] The following define specific terms, as they are understood
to be used in the present disclosure.
[0057] By the term "polymer dope (s)" or "dope (s)," we mean a
polymer dissolved in a solvent, or in a miscible
solvent/non-solvent combination such that polymer dope (s) will
form a pore structure when subjected to a phase inversion
process.
[0058] By the term "phase inversion process," we mean a process of
exposing a polymer dope to a controlled environment that provides
for controlled interdiffusion of dope, solvent, and/or nonsolvents
into or out of the membrane in response to phase inversion so as to
induce liquid-liquid demixing so as to form a pore structure. Phase
inversion is a necessary step in the formation of microporous
membrane. The process is induced by a number of mechanisms well
known to those versed in the art. Examples of phase inversion
include but are not limited to: contacting the polymer dope coating
to a solution of solvent and nonsolvent containing a higher
percentage of nonsolvent than the dope solution; thermally induced
phase inversion; exposing membrane to a vapor interface and
evaporating the solvent from the dope coating.
[0059] By the term "coating surface," we mean a very smooth flat
essentially impermeable non-porous surface support that the dope
will wet but from which the wet microporous multilayer membrane
precursor will readily release after the phase inversion process.
Suitable non-porous surface support coating surfaces can be, such
as, for example, a belt or a drum, disposable or reusable, and made
from materials such as PET film or stainless steel. We would
anticipate that a flexible coating surface would require additional
support (for example rollers underneath a smooth flexible belt) to
properly position the coating surface at the point(s) of
application of the multiple polymer solution coatings.
[0060] By the term "unsupported multilayer membrane," we mean
membrane without an integral porous support or scrim in which each
dope layer is applied to the coating surface by one or more
pre-metering dope applying or coating apparatus. The resulting
layers are subjected to a phase inversion process, separated from
the support, washed and then dried. Suitable processes for the
washing and drying are well known in the art. References to the wet
microporous multilayer membrane precursor characterize the
intermediate product, after phase inversion, but just prior to the
washing and drying step.
[0061] By the term "cooperatively applying polymer dopes," we mean
that the multiple coating dope layers form cooperatively in such a
manner that there is hydrodynamic equilibrium i.e., there is no
significant interfacial shear turbulence between the two liquid
layers.
[0062] By the term "dope applying apparatus," we mean pre-metering
devices that operatively transfers polymer dope to a coating
surface such that the thickness of the transferred dope is
substantially controlled through control of the solution feed rate
and the coating surface speed and is not dependent upon the gap
between the coating surface and the pre-metering apparatus.
Examples of such devices include, but are not limited to, slot
dies, extrusion dies, slide dies, or curtain dies and other
structures which prove capable of performing the function of the
representative examples above.
[0063] By the term "substantially, simultaneously coating multiple
fluid layers," we mean that multiple dopes are applied to the
coating surface in such a manner so that no significant evaporation
of solvent is allowed to occur between the application of each
succeeding dope layer. This limiting of solvent evaporation is best
achieved by using low volatility solvents.
[0064] By the term "acceptable Forward Flow Bubble Point (FFBP)
curve for a multilayer membrane" we mean a curve that essentially
conforms to the Type I characteristics as shown in FIGS. 16 a and
16 c, demonstrating the presence of distinct and symmetric pore
size distributions in each of the membrane layers as opposed to the
clear transition zone illustrated by a Type II characteristics as
shown in FIGS. 16 b and 16 d.
[0065] As part of ongoing efforts to more fully understand the
phenomenon of the phase inversion formation of microporous
membrane, applicants have recently recognized, through the
continuous evaluation of the experimental data, including the data
disclosed in the parent application, and astute observation thereof
that multilayer unsupported microporous membrane made with
post-metering devices appears to have a transition layer with pore
sizes that are different from those of the two constituent layers
at the interface between the two layers and that this is believed,
at least partially, caused by shear turbulence forces resulting
from the post-metering application of each layer.
[0066] As shown in FIG. 1, a representative system 10 for producing
an at least a two-layer, multilayer unsupported microporous
membrane is illustrated. As shown, the representative system
comprises a representative pre-metering device, such as, for
example, a slot die (schematically shown) having at least two feed
slots 14, 16. The die has an upstream die lip 18 and a downstream
die lip 20. When dope is being applied to a moving coating surface
or web 22, and dopes A and B are exiting their respective feed
slots 16, 14, at controlled pumped flow rates, there is no
significant shear turbulence being produced between the support
surface 22 and the first polymer dope solution (B) layer, or
between the upper interface of the first polymer dope solution (B)
layer and the lower interface of the second polymer dope solution
(A) layer and two static contact lines 24, 26, are formed as
illustrated. As shown, when the coating surfaces or web 22 is
moving in the direction as shown for web direction, dope B, which
is being fed from a location by any one of a plurality of known
means, is deposited on the coating surface and forms an upstream
meniscus 28 and a down stream meniscus 34 at a dynamic contact line
29. As the coating surface 22 moves, the dope B that is deposited
thereon is also moving at essentially the same speed and in the
same direction as the coating surface so that there is no
significant shear turbulence produced between the polymer solution
B and the coating surface in the direction of the web. An
additional dope A from the second feed slot 16 is fed, with the
feed slots being divided by a center die lip 30 is then applied
onto the top of the first dope B for a distance, shown as the
coating gap 32, between the downstream die lip 20 and the dynamic
contact line 29. The application of at least this second dope A
from at least the second feed slot 16 is conducted at a flow rate
such that interfacial shear turbulence is essentially eliminated
between the two layers A, B. Once these layers A, B are in intimate
contact at the interface 32, there is a molecular level
intermingling of the extended polymer chains that results in the
subsequent formation of a continuous polymer structure during phase
inversion. The absence of shear turbulence at the interface
prevents any gross mixing of the two polymer solutions. The
interaction at the molecular level is described in U.S. Pat. No.
6,090,441 of Vining et al., the disclosure of which is herein
incorporated by reference to the extent not inconsistent with the
present disclosure, with the first dope A from feed slot 14 at an
interfacial region 32.
[0067] At this point, the motion of the coating surface 22 moves in
one direction and carries the polymer dopes A, B from the
downstream meniscus 34 to a phase inversion process step (not
shown), as is known in the art. As schematically illustrated, there
is a separation line 38 between the at least two dopes A, B formed
proximate the down stream wall 39 of the first dope feed slot 14.
Further, in the interfacial region 32, it has been observed that
the interfacial dope region (region 32) illustrates a clear
demarcation in pore size but the polymer structure is continuous.
As shown, the coating gap 33 is adjustable and will be controlled,
as is known in the art.
[0068] The resulting pore size of the each of the layers made, as
shown in FIG. 1, will be predetermined by the formulation and/or
thermal history of each polymer dope It is expected that viscosity
will need to be controlled for the proper functioning of whichever
pre-metered coating apparatus is chosen. It is also anticipated
that the distance from the die slots to the receiving coating
surface must be controlled. Too small a distance will result in
mechanically induced shear turbulence in the uppermost polymer
solution layer and too great a distance will break the membrane
meniscus and lead to defects in the surface and poor control.
[0069] The process presently envisioned may involve exposing one or
more of the polymer dope layers to an air gap. These exposure can
occur (1) if two separate coating apparatus are used in series and
(2) during the period in which the multilayer coatings are
transported on the moving support from the coating step to the
phase inversion step. The presently preferred approach to dealing
with an air gap so as to avoid solvent evaporation and resulting
skin formation consists of using a very low vapor pressure (low
volatility) solvent in the polymer solution formulation. If this
approach is not feasible, then the gaseous atmosphere in the air
gaps may be controlled to maintain an appropriate solvent partial
pressure directly above the coated polymer solutions, as is known
in the art.
[0070] The above disclosure applies to those polymers, copolymers,
and polymer mixtures that are capable of forming a microporous
membrane by means of one or more of the well known variants of the
phase inversion process, such as, for example, nylon, PVDF, PES, or
polypropylene may function. It is not mandatory that the two (or
more) dope layers be subject to thermal manipulation if dope
formulation can alter effective pore size.
[0071] While it was initially anticipated in the parent application
that a single die with multiple slots would function more
effectively than separate dies, actual experience has proven that
two or more single slot dies separated by a small distance have
also proven to be operable. As shown by the examples, it has proven
possible to use a two slot die in combination with an at least one
additional slot die to make a three layer membrane and thus, four
or more layer membranes appear possible as well.
[0072] Prior to the conduct of the examples below, it was believed
that a single multiple slot die was needed to produce the multiple
cast dope layers. Experience has proved this theory to be overly
limiting according to the examples below. Using pre-metering
coating apparatus for applying a second coated dope layer to form
one layer of the membrane in a separate step from coating the first
dope layer onto the support has avoided interfacial shear
turbulence and mixing, as shown by the examples below. In addition,
physically separating the casting of the dope layers in space from
the phase inversion formation of the membrane has also not proven
to lead to significant solvent evaporation from the first coated
layer, which might lead to interlayer skin formation, provided a
low volatility solvent is used.
[0073] It has now been determined, upon careful review of all the
data compiled prior to the filing of the present application, that
a single pre-metering coating apparatus, such as, for example, a
slot die with multiple slots would be capable of achieving the
desired unsupported multilayer microporous membrane structure. We
have now found, after careful reflection and review of the
experimental results conducted prior to the filing of the parent
application, that two or more physically separated single slot
pre-metering coating apparatus are also capable of producing these
desired results, such as, for example, slot dies.
[0074] The one or more pre-metering coating apparatus, such as, for
example, a multiple slot die would, presently preferably, be
mounted vertically above a horizontal coating surface such that the
polymer dope is applied downward but, conceivably other
orientations could be employed and it was determined that vertical
orientation of the coating surface operatively functioned to
produce satisfactory unsupported multilayer membrane.
[0075] FIGS. 2 and 3 illustrate various possible embodiments of the
unsupported multilayer membrane according to the teachings of the
present disclosure. As shown, Type I illustrates an unsupported
multilayer membrane wherein the pore sizes are different such that
the larger pore size membrane would serve as an upstream protection
layer for the smaller pore size membrane extending the life of the
filter media.
[0076] As shown, Type II illustrates a simpler case of Type I
wherein the two pore sizes are the about the same. This approach
may be preferred over a single cast membrane since membrane
composed of multiple coatings reduces the risk of a single defect
in the single coating of a single cast membrane compromising the
overall retention of the membrane. Further since by following the
teaching of the present disclosure, shear turbulence during
multilayer membrane production is essentially eliminated, in those
applications which might require the same or nearly the same pore
size but where different additives are desired present in the
different layers, multilayer microporous membrane can be produced
with little, if any mixing of the dopes due to shear
turbulence.
[0077] As shown, Type III illustrates the reverse of Type I in the
event that the reverse coating sequence confers some end-use
advantage for either the upstream or the downstream side of the
finished membrane.
[0078] As shown, Type IV illustrates an unsupported multilayer
membrane wherein two outer larger pore-size layers sandwich a
middle layer having a relatively smaller pore size. Such
constructions are advantageous because the outside layers protect
the inner qualification layer from damage during filter cartridge
fabrication.
[0079] As shown, Type V illustrates an unsupported multilayer
membrane wherein the two outer smaller pore size membrane layers
sandwich a larger pore size inner or middle layer. Such a
construction may provide the retention benefit of a small pore size
membrane but yield a higher permeation rate than a conventional
design since the inner membrane layer has a larger pore size and
therefore exhibits a lower pressure drop.
[0080] As shown, Type VI illustrates an unsupported multilayer
membrane wherein three layers are stacked with progressively
decreasing pore size.
[0081] As shown, Type VII illustrates the situation wherein the
largest pore size layer is initially positioned against the coating
surface, if such location should prove advantageous.
[0082] As is apparent to anyone skilled in the art, additional dope
layers to form additional membrane layers can be added up to the
practical limit of membrane fabrication without substantially
deviating from the spirit of the present disclosure.
[0083] In addition, we envision membranes wherein the membrane
layers could be varied by properties other than pore size, such as
by chemistry or molecular weight. In some cases, lower molecular
weight polymers offer a high degree of functionality but do not
offer strength.
[0084] A type VIII configuration could be made by producing
membrane layers by combining dopes from polymers of differing
molecular weight. Logically, this concept could be applied to three
or more membrane layers.
[0085] A type IX membrane could be made if the dope layers varied
in polymer chemistry. For example, in PVDF membrane, the relative
amount of polyvinyl pyrrolidone can be varied to adjust properties.
Logically, this concept could be applied membrane having three or
more layers.
[0086] As is apparent to anyone versed in the art, additional
membrane layers can be added up to the practical limit of membrane
fabrication without substantially deviating from the spirit of this
disclosure.
[0087] The following represents actual experiments conducted to
verify the concept described above.
PVDF Experiments:
[0088] Originally in the parent application, the following
experiments were conducted to confirm the viability of the
producing multilayer, unsupported microporous membrane from PVDF
Polymer using post-metered casting process (knife casting).
However, upon further review of the original results, the following
experiments were found to demonstrate the characteristics of single
and multilayer unsupported microporous membrane made using PVDF
Polymer and post-metered casting process (knife casting), which
were initially not appreciated.
PVDF Ingredients
[0089] The following ingredients were employed in the experiments
that follow. TABLE-US-00002 Chemical Trade name Manufacturer Mfg.
Location PVDF Kynar 761 Elf Atochem Philadelphia, PA NA IPA
2-propanol ACS Aldrich Milwaukee, WI reagent NMP 1-methyl 2
pyrrolidinone Aldrich Milwaukee, WI ACS reagent
PVDF Methods
[0090] A mixture of 15%, PVDF (Kynar 761), 15% IPA (2-propanol ACS
reagent) and 70% NMP (1-methyl 2 pyrrolidinone ACS reagent), with a
total weight of about 200 grams, were blended and sealed into a jar
with a magnetic stir bar, all of which was immersed into a jacketed
beaker with the water in the jacket circulated at a predetermined
temperature. A magnetic stirrer provided the mixing. The resulting
dope was heated to a temperature between about 10-15.degree. C.
below the target temperature (T.sub.max), to bring the constituents
into solution.
[0091] The dope was then heated to the target temperature
(T.sub.max) via the circulating water in the jacket and held at
that temperature for a minimum of about one hour.
[0092] To make single layer membrane, the dope was poured into a
post-metering casting apparatus, in this case, a doctor blade with
a gap setting of about 0.016 inches and pulled across a substrate,
in this case, a piece of glass, at about 6 feet per minute to cast
the membrane film. If a two layer membrane was made, a second dope
was placed into a second doctor blade with a gap of about 0.032
inches. The second doctor blade was made wider than the first
doctor blade and the back plate was raised so that the sides and
back of the second doctor lade would not drag through the already
cast layer of the first cast polymer dope and disrupt the first
cast dope surface. There was approximately a 30 second delay
between the casting of the first layer and the second layer of
dope.
[0093] The glass plate with the cast polymer dope, one or two
layers, was then submerged into a shallow tray containing a quench
fluid of about 25% deionized water and about 75% isopropyl alcohol
thereby inducing phase inversion. After about three minutes the
glass plate and wet microporous membrane precursor were removed
from the quench solution and transferred to rinse. The rinse
consisted of a shallow tray of flowing deionized water with some
overflow to help flush out impurities. The wet microporous membrane
precursor was then lifted from the glass plate and allowed to rinse
for a minimum of about 30 minutes to ensure complete removal of
solvents. The quenched membrane was then restrained on a hemidrum
in a drying fixture and dried in a convection oven at about
70.degree. C. for about thirty (30) to about forty (40) minutes.
TABLE-US-00003 TABLE 1 Experiments with PVDF Membrane Made via
Post-Metering Process Avg. Flow Membrane Avg. IPB Avg. FAOP @ 5
psid Thickness ID Construction psi psi ml/min/cm.sup.2 mils
0410s67.5 Two layers 12 16.5 7.1 8.2 0410s62.5 Two layers 21 26 3.9
7.7 0228sdr67.5 Two layers 36.5 53 0.8 9.4 0508lam55 2 layers
laminate 43 58 1.1 6.3 0123-62.5A-22 single layers 8.25 10.5 19.3
6.2 0223E single layers 22.8 38 1.1 7.5 0119-60A-24 single layers
33.3 38.5 1.5 6.2
[0094] Table 1 summarizes some of the examples of membranes
prepared with sequential applications of PVDF membrane as described
in the above procedure and provides a comparison between single and
double layer membranes. An example of a laminated membrane is also
provided wherein two separate and distinct membranes were
separately cast, quenched and rinsed prior to being pressed and
dried together, in accordance with one prior art process, to form
the laminated sample. The controls for this example consisted of a
single layer membrane. It should be noted that only a
representative number of examples are presented for brevity
purposes, as a number of other example experiments were actually
conducted.
[0095] At a given bubble point, the expected result was that a two
layer membrane would yield better flow than a single layer
membrane. This expected result was based on the theory that the
thickness of the relatively small pore size layer in a two layer
membrane would be less than the total thickness of a single layer
membrane (the value in Table 1 is total thickness) and since flow
is a function of thickness, flow was expected to improve.
[0096] As will become apparent, Scanning Electron Microscope (SEM)
and forward flow bubble point analysis make it clear that a two
layer membrane construction was achieved. However, the anticipated
improved flow for a two layer membrane relative to the prior art
was difficult to compare due to variations in bubble point of the
samples.
[0097] FIG. 4 illustrates a cross section of an unsupported
multilayer PVDF membrane cast with a dope applying apparatus, such
as, for example, a doctor blade as described in the above
procedure. The T.sub.max of the dope used to form the relatively
large pore size layer was 67.5 C and the bubble point was
approximately 4-5 psi in 60/40 IPA/water. The dope used to form the
relatively small pore size layer had a T.sub.max of about 55 C and
a forward flow bubble point of about 40 psi. The initial bubble
point of about 36.5 psi, as shown in Table 1, was a little
lower.
[0098] It is anticipated that a two layer membrane or more
multilayer membrane will offer better filtration life than a single
layer membrane of the same bubble point and thickness. As can be
readily seen, two distinct layers having distinct pore sizes are
clearly visible. Further, FIG. 4 illustrates a distinct
differentiation between one pore size layer and the other pore size
layer, while the membrane itself is continuous in that it has a
unitary structure. Thus, FIG. 4 clearly illustrates that a
multilayer unsupported microporous membrane having at least two
different pore sizes can be produced, in accordance with the
present disclosure.
[0099] FIG. 5 illustrates another cross section of an unsupported
multilayer PVDF membrane wherein the T.sub.max of the dope used to
produce the open or relatively large pore size layer was about
67.5.degree. C. and the T.sub.max of the dope used to produce the
tight or relatively small pore size layer was about 62.5.degree. C.
FIG. 6 illustrates a close-up of the interface of the two layers to
show that the transition between the layers appears to be seamless,
i.e., continuous. Although the pore size changes from one layer to
the next, the membrane formed has a continuous interface between
the two adjacent layers.
[0100] FIG. 7 illustrates two forward flow bubble point curves for
sample 0228sdr67.5. The data for FIG. 7 was generated by testing
the sample with the open or relatively large pore size side up, and
then flipping the sample over and testing the sample again with the
tight or relatively small pore size side up.
[0101] With the sample membrane's open or relatively large pore
size side up, the rising air pressure will first clear the pores of
the open or relatively large pore size layer. The fluid then clears
the open or relatively large pore size layer but the tight or
relatively small pore size layer underneath retains the fluid until
an adequate pressure is reached to clear those pores. The mass flow
meter records this first event either with a temporary increase in
airflow (shown as a peak) or a permanent increase in airflow that
continues at a low level below the level of bulk flow. This latter
phenomenon appears to be at least partially due to the increase in
diffusion flow because the gas must only diffuse through half the
membrane or one layer.
[0102] While it was clear from the SEM photograph of FIGS. 5 and 6
showing sample 0228sdr67.5, the forward flow bubble point graph of
FIG. 7 illustrates the results of a forward flow bubble point test
using a much larger sample of the membrane as contained in a 90
millimeter disc. These results clearly indicated that the membrane
produced and tested was a multilayer membrane, with the multilayer
structure being achieved over the entire 90 millimeter disc surface
area, thus, confirming that the membrane produced was, in fact, a
multilayer membrane and thereby confirming that the technique of
production used in the examples were both practical and
effective.
[0103] FIG. 8 illustrates a forward flow bubble point curve for a
prior art laminated membrane. Like the unsupported multilayer
membrane of the present disclosure, this forward flow bubble point
curve also exhibits the peak at 5 psi when the upper layer pore
clears. Thus, it is clear from FIGS. 7 and 8 that the presence of a
peak clearly indicates that a multilayer membrane is present and
that the membrane of FIG. 7, when produced, was actually a
multilayer membrane as compared to the control of the two
separately formed pore size membranes laminated together of the
prior art.
[0104] FIG. 9 illustrates a single layer membrane forward flow
bubble point curve. Note that the peak at 5 psi is missing
regardless of sample orientation of the membrane in the test stand.
There is no difference between the curves. Thus, it should be clear
that there is no peak discernable from the forward flow bubble
point for a single layer membrane. Therefore, the appearance of a
peak in the forward flow bubble point graphs, as illustrated in
FIG. 7, clearly indicates that the membrane tested was, in fact, a
multilayer membrane.
[0105] FIG. 10 illustrates a cross section of the laminated PVDF
membrane. In the close up of the same membrane shown in FIG. 11, it
is clearly ascertainable that the laminate does not form a
continuous interface between the two layers, but each layer is
simply pressed into place relative to the other layer. This type of
bond between the layers is not as inherently strong as the
continuous two layer membrane shown in FIG. 6.
[0106] The above example clearly demonstrates that multilayer
unsupported microporous membrane has been produced using
polyvinylidene fluoride (PVDF) according to the concepts presented
in the present disclosure.
[0107] Prior to explaining the significance of the test results in
terms of FEBP, FIGS. 16a-16d will be discussed. FIG. 16 a
illustrates a cross section of a multilayer microporous membrane
200 produced using pre-metering coating apparatus (not shown). As
shown, the membrane 200 has a large pore size layer 202 and a
relatively smaller pore size layer 204. As can readily be seen, the
pore size distribution 206 shows a distinct demarcation line
between the two layers 202, 204 at 206.
[0108] The FFBP test results of the membrane 200 is graphically
illustrated in FIG. 16 c and the distinct demarcation line between
the two layers 202, 204 at 206 is evident by the large pore and
small pore bubble points at the positions shown and the slope of
the line connecting same.
[0109] FIG. 16 b illustrates a cross section of a multilayer
microporous membrane 200 produced using post-metering coating
apparatus (not shown). As shown, the membrane 230 has a large pore
size layer 232 and a relatively smaller pore size layer 234. As can
readily be seen, the pore size distribution 236 shows a distinct
transition zone between the two layers 232, 234 at 238.
[0110] The FFBP test results of the membrane 200 is graphically
illustrated in FIG. 16 d and the distinct transition zone between
the two layers 202, 204 at 206 is evident by the large pore and
small pore bubble points at the positions shown and the slope of
the line connecting same.
[0111] Comparing FIG. 7 with FIGS. 8 and 9 however, reveals that
the transition between layers, induced by the doctor blade, caused
a Type II FFBP curve as illustrated in FIG. 16 d and demonstrated
in FIG. 7. This response was not evident when the membrane was
tested in reverse. Therefore, while a doctor blade provides a
continuous interface between the layers, a doctor blade also
produces an asymmetric interfacial transition zone. This phenomenon
is not obviously revealed by SEM but can be readily seen with a
FFBP analysis. Of the various examples generated with the doctor
blade, FIG. 7 represents the best performance obtained using the
post-metering apparatus and all other such examples run yielded
even greater differences between the slopes generated with opposite
membrane orientations.
Nylon Experiments
[0112] The following experiments were conducted to confirm the
viability of the producing multilayer, unsupported microporous
membrane using Nylon.
Nylon Ingredients
[0113] The following ingredients were employed in the experiments
that follow. TABLE-US-00004 Chemical Trade name Manufacturer Mfg.
Location PET film CI-100 500 gauge FilmQuest St. Charles, IL Nylon
6,6 Nylon 66Z or Solutia St. Louis MO 66B Formic Acid Formic Acid
BP Amoco Cleveland, OH Methanol Methanol Borden & Remington
Fall River, MA Chemical Co.
Nylon Methods Preparation of the Dopes:
[0114] Two nylon dopes were prepared using the methods described in
U.S. Pat. No. 4,707,265, Example 1. The dopes were produced using
about 16.0 percent by weight Nylon 66 (Solutia Vydyne.RTM. 66Z)
polymer.
Process Description
[0115] Geometrically symmetric and pore size symmetric unsupported
two and three layer membranes, each with their own pore structure
was prepared as follows.
[0116] As illustrated in FIG. 15, a Polyester film suitable for use
in the preparation of the present innovative unsupported multilayer
membrane (commercially available from FilmQuest St. Charles, Ill.
as part number CI-100 500 gauge), was conveyed past both a single
slot (slot C) and a multiple slot die (slots A and B), with all
slots (A, B and C) of the slot dies being located on the same side
of the PET film at speeds of about 20 ft/min.
[0117] When three layers of dope were coated on the PET film, the
dope from the first slot, (slot C), was applied at a weight of
about fifteen (15) gm/sq. meter of nylon solids. The dope from the
other two slots (slots A and B) was coated at a weight of about
twenty (20) gm/sq. meter of nylon solids.
[0118] If only two of the slots of the two slot dies were used, the
dope from both slots was coated at about twenty (20) gm/sq. meter
of nylon solids regardless of which of the two dies was used. The
nylon solids were provided from the dissolved nylon in the dope
solution, which was, for this example, sixteen (16.0) wt. % nylon
solution.
[0119] Almost immediately following the application of the first
dope layer, when a dope was cast using the first die, one or two
other layers were cast from the double slot die on top of the first
coated layer, first with a dope that produced a different pore size
and then with a second dope that produced a different pore size of
the two dopes or when all three slots were used, the three dopes,
as shown in Table 2 below. The distance between the slots A and B
in the multislot die was about 15-20 mils. The distance between the
slot of the first die and the second slot of the multislot die was
about 9.5 inches.
[0120] In one representative example, the coating weight of the
dope delivered from each slot of the multislot slot die was about
twenty (20) gm/sq. meter of Nylon solids in about a sixteen (16.0)
wt % solution. The thus coated three dope multilayer structure was
then quickly brought into contact with a Marinacco-style quench
solution, which simultaneously quenched the multilayer structure
from the outer surface of the multilayer structure furthest from
the PET film, such that a multilayer, continuous microporous
membrane structure was formed.
[0121] In both the production of a two layer membrane or a three
layer membrane, the quenched membrane was then washed, hand peeled
from the PET film just after it was rinsed, mounted and restrained
on a hemidrum and then dried. Removing membrane from the film prior
to drying was found to be advantageous.
[0122] The test results are shown in Table 2 below. TABLE-US-00005
TABLE 2 Results of Nylon Trials BP m 60/40 IPA/Water psi BP PET
Film Middle Quench Sample response slot C slot B slot A Layers 0103
predicted 15 22 35 3 0103 actual 12 23 29 0103 difference 3 -1 6
0107 predicted 15 35 2 0107 actual 15 30 0107 difference 0 5 0110
predicted 15 22 2 0110 actual 15 19 0110 difference 0 3 0111
predicted 15 22 2 0111 actual 15 19 0111 difference 0 3 0206
predicted 15 22 2 0206 actual 15 21 0206 difference 0 1 0207
predicted 15 22 35 2 0207 actual 18 18 24 0207 difference -3 4 11
0209 predicted 22 15 2 0209 actual 20 16 0209 difference 2 -1
[0123] Table 2 illustrates attribute testing of the unsupported
multilayer nylon membrane produced as described above. Most samples
were run as two layer membrane and provided a two layer microporous
membrane structure as evidenced by the forward flow bubble point
curves, which show higher diffusion rates when the upper layer of
pores have cleared, and SEM photographs. The unsupported multilayer
structure was evident whether the dies were run in sequence or
simultaneously. The samples identified above as 103 was run as a
three layer membrane.
[0124] FIG. 12 show forward flow bubble point curves for nylon
membrane wherein pressure is ramped continuously on a membrane
wetted with about 60% IPA and about 40% water and the flow was
monitored with a mass flow meter. As is known, flow is a
measurement of either diffusion through the wetted membrane or bulk
flow through the cleared pores or a combination.
[0125] When a membrane consisting of a single layer was tested, the
response curve was independent of orientation, as illustrated above
for the PVDF membrane. However, when an unsupported, multilayer
membrane of the present disclosure was tested, the response curve
differed, depending on whether the larger pore size layer was
upstream or downstream relative to the smaller pore size layer. If
the larger pore size layer was upstream, when the pressure
necessary to clear those pores was reached (the bubble point), the
larger pore size layer suddenly cleared. At this point, the liquid
will progress down until the smaller pore size layer just beneath
the larger pore size layer is reached. However, once the pores of
the larger pore size layer has cleared, the diffusion response also
increased because the air no longer must diffuse through the entire
depth of the membrane, but only through half of the membrane, the
smaller pore layer.
[0126] On a forward flow bubble point (FFBP) curve, this transition
causes an increase in the mass flow response. If a membrane, was
tested with the relatively smaller pore size layer toward the air
interface, then the pores will not clear until the relatively
smaller pores have reached their bubble point, at which time the
entire membrane clears. Since the membrane remained fully wetted
during the entire test, the diffusion does not increase during the
latter part of the test.
[0127] This difference is best illustrated in FIG. 12 wherein two
curves are displayed for the same membrane sample. As shown, when
tested with the relatively larger pore size layer upstream, the
mass flow rose above the baseline at the bubble point of the
relatively larger pore size upstream layer but did not experience
bulk flow until the relatively smaller pore size pores were cleared
as well.
[0128] Membrane sample 103 was a three layer membrane. As can be
determined from both the SEM photograph of FIG. 13 and the forward
flow bubble point curve of FIG. 14, three distinct membrane layers
can be ascertained. The first layer was measured at about 13 psi
from the curve generated with the open or relatively large pore
layer upstream. This measurement can be seen as a significant rise
above the baseline curve, which is generated with the tight or
relatively small pore layer upstream. The second layer cleared at a
pressure of about 24 psi, where the curve once again rises above
the baseline. The third layer is not apparent in the curve where
the open or relatively large pore layer side was placed upstream in
the test. However, the third layer is apparent at 29 psi in the
baseline curve when that same piece of membrane was flipped over
and tested.
[0129] FIG. 15 schematically illustrates one possible
representative apparatus that could be used with one possible
representative method to produce the innovative multilayer,
unsupported membrane of the present disclosure.
[0130] As shown, the apparatus, similar to that disclosed in U.S.
Pat. No. 6,090,441 to Vining et al., the disclosure of which has
been previously been incorporated by reference herein, includes a
casting or coating surface, PET film, used as the base upon which
the dopes are deposited by a series of slot dies, a single die and
then a multiple die, it being understood that other die or dope
applying apparatus arrangements could be used, as well as different
coating surface orientations, as long as the interfacial shear
turbulence and interlayer mixing are eliminated or minimized and
the innovative unsupported, multilayer membrane is successfully
produced.
Prophetic Example PES Membrane
PES Composition Ingredients
[0131] The following ingredients are employed in the composition of
PES dope. TABLE-US-00006 Recipe 1 (approximately 0.4 .mu.m pore
size) Chemical Amount Poly ether sulfone 14% 1-methyl 2
pyrrolidinone 18% Poly ethylene glycol 400 66% water 2%
[0132] TABLE-US-00007 Recipe 2 (approximately 0.8 .mu.m pore size)
Chemical Amount Poly ether sulfone 13% 1-methyl 2 pyrrolidinone 19%
Poly ethylene glycol 400 67% water 1%
PES Methods
[0133] Two PES dopes are prepared per recipes 1 & 2 by mixing
the components of each dope in separate agitated, jacketed tanks.
Each dope is heated, under agitation to about 70.degree. C. for
about two hours and then cooled to ambient temperature.
Process Description
[0134] As illustrated in FIG. 15, a polyesterpolyester film
(commercially available from FilmQuest St. Charles, Ill. as part
number CI-100 500 gauge) suitable for use in the preparation of the
present innovative unsupported multizone membrane, is conveyed past
both a first slot (slot C) and a second slot die (either slots A or
B), with all slots (A, B and C) of the slot dies being located on
the same side of the PET film at a speed of about 1 ft/min.
[0135] When two layers of dope are coated on the PET film, the dope
from the first slot, (slot C), is applied at a weight of about
twenty (20) gm/sq. meter of polyether sulfone solids. The dope from
the other slot (either slot A or B) is coated at a weight of about
twenty (20) gm/sq. meter of polyether sulfone solids.
[0136] The distance between the slots A and B in the multislot die
are about 15-20 mils. The distance between the slot of the first
die and the second slot of the multislot die is about 9.5
inches.
[0137] The coated two dope multilayer structure is then brought
through a humidification chamber for about 15 to about 20 minutes
where the humidity is initially about 90% and gradually drops to
about 50% at the end of the chamber. The dope will be in the form
of a phase inverted membrane when exiting the chamber that is not
yet washed and dried.
[0138] In the production of a two zone unsupported membrane the
nascent membrane is then rinsed, separated from the PET film just
after the rinse, and then is dried.
[0139] Thus, it is clear from the above that the present disclosure
discloses innovative apparatus, methods and membrane that solve the
prior art difficulties with the production of unsupported,
multilayer microporous membrane.
[0140] While the articles, apparatus and methods for making the
articles contained herein constitute preferred embodiments of the
disclosure, it is to be understood that the disclosure is not
limited to these precise articles, apparatus and methods, and that
changes may be made therein without departing from the scope of the
disclosure which is defined in the appended claims.
* * * * *