Field of the Invention
The invention relates to a method for the production of microporous
polyvinylidene fluoride membranes. In particular, the invention relates to the
production of microporous polyvinylidene fluoride membranes that differ significantly
in both their structural and functional characteristics from conventional polyvinylidene
fluoride microporous membranes.
Background of the Invention
Polyvinylidene fluoride microporous membranes, generally formed as
thin sheets of substantially uniform thickness, have a sponge-like internal structure
containing millions of channels. These channels define a tortuous flow path for
liquids from one side of the membrane sheet to the other side. Conventional methods
of producing these polyvinylidene fluoride (hereinafter "PVDF") membranes result
in membranes having a matrix of intercommunicating channels, the channels having
a substantially uniform width within narrow limits.
Microporous membranes act as screens or sieves and retain on their
surface all particles larger than the given width of a channel (i.e. pore diameter).
Entrapment on the membrane surface of particles of approximately the pore diameter
will rapidly plug the membrane irreversibly, leading to rapid decline in flow rate.
Due to the tortuous nature of the flow channels in conventional microporous membranes,
significant hydraulic pressure is needed to force liquids from one side of the
membrane to the other. As the membranes clog, this pressure necessarily increases.
Conventionally-produced PVDF membranes are commercially available
with average pore sizes (i.e. pore diameters) in the range from about 0.10 µm to
about 5.0 µm. The smallest of these conventional pore sizes will retain some large
viruses and most bacteria. However, most viruses and some bacteria are not retained.
In addition, the smallest of these conventional pore sizes will not retain large
macromolecules. Attempts to produce microporous membrane filters having pore sizes
less than about 0.10 µm have generally led to problems of very slow flow because
of the small pore size, and to problems of rapid plugging.
Conventional solvent-casting procedures for producing microporous
PVDF membranes rely on the use of a solvent such as acetone for the PVDF polymer.
Nevertheless, acetone is usually not thought of as a solvent for this particular
polymer because it is exceedingly difficult to dissolve any appreciable quantity
of PVDF in acetone at room temperature. In order to dissolve a sufficient quantity
to form an adequately viscous solution for use in practising conventional methods,
the acetone must be heated close to its boiling point of about 50°C. This produces
severe constraints on conventional methods since the initial mixing of PVDF must
occur at an elevated temperature.
Examples of prior art methods of forming microporous membranes are
the methods disclosed in WO-A-91/17204, US-A-3642668, US-A-4946889, JP-A-63296940,
US-A-4399035, US-A-4203847, EP-A-0233709 and DE-A-2632185.
US-A-4399035 discloses hollow fibre PVDF membranes prepared by solution
casting and coagulation of the film with non-solvent. Mixtures of good and "poor"
solvents may be used in the casting solution. Water, MeOH and EtOH are disclosed
as coagulating baths liquids, but there is no mention of adding a poor solvent
to the coagulating bath.
JP-A-63-296940 discloses PVDF membranes prepared by solution casting
and coagulation of the film with non-solvent. The casting solutions comprise DMSO
and compounds labelled as "non-solvents", such as water, formamide, butanol, ethylene
glycol, glycerine, urea or CaCl2, or polyethoxylated surfactants. Water
is mentioned as coagulant.
US-A-4 203 84 discloses PVDF membranes prepared by solution casting
and coagulation of the film with non-solvent. According to the claims, acetone,
although not considered as a "good" solvent, is used as the preferred solvent for
casting. Mixtures of acetone and water are used as formation bath liquids. Mention
is made of methanol and ethanol as possible nonsolvents for the formation bath
US-A-4 203 847 discloses PVDF membranes prepared by solution casting
and coagulation of the film with non-solvent. Acetone is used as the preferred
solvent. Mixtures of acetone and dimethylformamide are mentioned as possible casting
solution solvents (col.18, l.18). Various mixtures suitable as formation bath liquids
are mentioned (col.18, l.19-23).
DE-A-26 32 185 discloses the preparation of PVDF (copolymers) membrane
prepared by casting a solution followed by coagulation with a non-solvent. The
casting solution may comprises two solvents with different solvating power, as
well as non-solvent. The only non-solvent mentioned as coagulating bath is water.
Inclusion of solvents in the coagulation bath is not addressed.
JP-A-4-100522 discloses hollow fibre PVDF membranes prepared by solution
casting and coagulation of the film with non-solvent. Mixtures of a main solvent
such as dimethylacetamide with a ketone-type co-solvent such as acetone and a non-solvent
are used in the casting solution. The coagulating bath comprises an aqueous solution
of the main solvent
US-A-3615024 discusses the wet phase inversion process for the preparation
of polymeric membranes and contains general remarks concerning suitable components
of casting solutions and coagulation baths. PVDF membranes are not mentioned.
Summary of the Invention
The present invention provides a process for making a family of microporous
PVDF membranes that differ substantially from conventional PVDF membranes in their
pore size, methanol bubble point, flow rate, and surface area.
In a first aspect, the invention provides a method of making a microporous
membrane as defined in claim 1
Particular and preferred aspects of the invention are as set forth
in the dependent claims appended hereto.
A microporous PVDF membrane formed by the process of the invention
has a flow rate of solution per unit area across the membrane that is substantially
greater (i.e. at least 30% greater) than the flow rate of a solution per unit
area in conventional microporous PVDF membranes of similar pore size and similar
thickness. PVDF membranes formed in accordance with the process of the invention
can also have an average pore size less than about 0.10 µm. Particular membranes
can have average pore sizes down to about 0.02 µm.
The microporous PVDF membranes formed in accordance with the invention
have a defined microstructure which includes intercommunicating flow channels extending
from one surface of the membrane to an opposite surface. At least one of the surfaces
of the membrane has a macro structure. In one embodiment, the macro structure is
defined by a plurality of substantially spherical features having a size substantially
greater than the average pore size. In another embodiment, the macro structure
is defined by a plurality of flattened modules covering substantially the entire
surface of the membrane.
In the method of the invention, a polymer is dissolved in a mixing
solution containing a solvent for the PVDF and at least one co-solvent, and the
dissolved polymer then is applied to a solid substrate as a thin film. The thin
film on the substrate is converted to a microporous membrane by displacing the
solvents from the thin film and replacing them with a nonsolvent for the PVDF
polymer. This occurs in a formation bath. The formation bath is a mixture of a
nonsolvent for the polymer plus said at least one co-solvent used in the mixing
One particular aspect of the process of the invention is the use
of a solvent:co-solvent system for the polymer, such as N-methyl-2-pyrrolidone:butylacetate
or N-methyl-2-pyrrolidone:formamide. The solvent:co-solvent system is adequate
to dissolve the polymer at temperatures between about room temperature and about
50°C. Another particular aspect of the process is that formation of the microporous
membrane in the formation bath can occur at substantially reduced temperatures.
In one embodiment of the invention, formation of the membrane of the invention
is preferably accomplished at 0°C.
The process involves forming a thin layer of a solution of the PVDF
polymer on a substrate, the PVDF dissolved in a solvent and co-solvent. The layered
substrate is then passed into a formation bath including at least the co-solvent
plus a nonsolvent, both the co-solvent and nonsolvent miscible with each other.
The layer of PVDF polymer is thereby transformed into a porous membrane, formed
on the substrate in the formation bath. The membrane is removed from the substrate
and then dried. A wide variety of structural and functional properties of the
membranes can be achieved in the process by selecting a temperature for either,
or both, the mixing solution and formation bath and also selecting a concentration
of co-solvent and nonsolvent for the formation bath.
Description of the Drawings
Detailed Description of the Invention
A. Microporous PVDF Membranes
- Fig. 1 is a scanning electron micrograph of the top surface of a conventional
microporous PVDF membrane. The bar represents a length of 10 microns;
- Fig. 2 is a scanning electron micrograph of the top surface of a microporous
PVDF membrane of the present invention. The bar represents a length of 10 microns;
- Fig. 3 is a scanning electron micrograph of the cross-section of the conventional
PVDF microporous membrane of Fig. 1. The bar represents a length of 10 microns;
- Fig. 4 is a scanning electron micrograph of the cross-section of the microporous
PVDF membrane of Fig. 2. The bar represents a length of 10 microns;
- Fig. 5 is a scanning electron micrograph of the top surface of another embodiment
of the PVDF membrane of the present invention. The bar represents a length of 10
- Fig. 6 is a scanning electron micrograph of the bottom surface of the conventional
microporous PVDF membrane of Fig. 1. The bar represents the length of a micron;
- Fig. 7 is a scanning electron micrograph of the bottom surface of the microporous
PVDF membrane of Fig. 2. The bar represents the length of 10 microns;
- Fig. 8 is a schematic diagram of the apparatus needed for a continuous method
of the present invention.
The present invention provides a process for preparing a family of
microporous PVDF membranes having nominal pore sizes ranging from about 0.02 µm
to about 2.0 µm. The term "nominal pore size" refers to the minimum size of particles
that will be retained on the membrane. Thus, a membrane with a nominal pore size
of about 0.45 µm means that particles greater than about 0.45 µm will be retained
on the membrane, those less than about 0.45 µm will pass through and will not be
The functional properties of these membranes differ significantly
from those of conventional PVDF membranes. Specifically, the present membranes
have greater liquid flow rates per unit area with equal particle retention as
compared to conventionally-made PVDF membranes of the same pore size and thickness.
This means that if a sample of a solution containing particles is passed through
the membrane and an equal volume sample of the same solution is passed through
a conventional membrane, both membranes will retain the same amount of material,
but the membrane obtained by the process of the present invention will have a faster
flow rate and process the liquid volume in a shorter time period. Moreover, compared
to conventionally-made PVDF membranes, the present PVDF membranes have an increased
surface area available at the point of contact between a molecule and the surface
of the membrane. This means that a sorted molecule is more strongly held to the
surface of present PVDF membranes. This is an advantage, for example, in immunodiagnostic
applications where the goal is to cause small amounts of expensive molecules such
as labelled reagents and tracers to be sorbed onto the surface of the membrane.
These functional advantages are a direct result of the unique structural characteristics
of the membranes of the present invention.
A scanning electron micrograph of a conventional PVDF microporous
membrane (Millipore Corporation, Bedford, Massachusetts) of 0.22 µm nominal pore
size is presented in Fig. 1. The surface morphology of this conventional PVDF
microporous membrane 10 is characterized by the well-known features of microporous
PVDF membranes; namely a foliate (i.e. leafy), substantially flat intertwined
mesh 12 of polymeric material with a pore size determined during manufacture. These
microporous membranes have a sponge-like matrix of intercommunicating passages
penetrating from one side of the membrane to the other side of the membrane. These
passages have a certain inner diameter, within narrow limits. The passages are
referred to as pores 14. The pores provide a tortuous flow path for liquids. In
conventional PVDF microporous membranes, the average width of the pores 14 as seen
on the membrane surface ranges from about 0.1 µm about 2.0 µm depending on the
Referring now to Fig. 2, the top surface of a microporous PVDF membrane
20 obtained by the method of the present invention having a substantially identical
pore size (0.22 µm) as the membrane of Fig. 1 is presented for comparison.
The top surface 22 shows a much more rounded aspect than does the
conventional membrane (Fig. 1). This microstructural morphology appears as a plurality
of globular bodies 23 attached to one another. The present membrane also has a
unique macrostructure. The term "macrostructure" refers to morphological features
of the present PVDF membranes that have dimensions substantially greater than
the average pore size. Fig. 2 illustrates that, in contrast to the conventional
PVDF membrane of Fig. 1, the top surface morphology of the present PVDF membrane
takes on a more cratered or pocked aspect, in which the top surface morphology
is arranged in roughly spheroidal, cratered constructions 24 having a scale size
of the order of about 3-4 µm.
As discussed below, microporous membranes obtained by the method of
the present invention are formed on solid substrates in one or more solvent-containing
formation baths. As used herein, the term "top surface" refers to that surface
of the microporous membrane that is not engaged with the solid substrate. Conversely,
the term "bottom surface" as used herein refers to that surface of the microporous
membrane that is engaged with the solid substrate during formation.
Without wishing to be bound by any particular theory, it is believed
that the particular macrostructural morphology of Fig. 2 is due to the orientation
of the PVDF molecule as the membrane is being formed in the process of the invention.
Conventional membranes, as exemplified in Fig. 1, are believed to contain fluorine
atoms as part of the PVDF molecules that are not ordered in any defined manner
but are randomly dispersed in several directions; thus resulting in the intertwining
surface configuration of Fig. 1. In contrast, the process of the present invention
is believed to orient the bulk of the fluorine atoms of the PVDF molecules toward
the outer surfaces (i.e. both top and bottom) of the forming membrane. This is
believed to result in the cratered appearance.
Furthermore, it is believed that the surface orientation of the PVDF
molecules results in a unique configuration of the middle of the membrane, which
has a lower concentration of PVDF per unit membrane as compared to the surface
regions. This means that less mass of PVDF is available to be converted into a
matrix of intercommunicating flow channels. Consequently, the middle portion of
the membrane is believed to have a less tortuous flow path.
Fig. 3 is a vertical cross section through the same conventional
microporous PVDF membrane as shown in Fig. 1. The interconnected surfaces 30 arranged
parallel to the plane of the photograph are numerous, are arranged in close proximity
to one another, and are densely packed in the interior of the membrane. It is believed
that the more numerous and closer together these surfaces, the more convoluted
the flow channels of the intact membrane.
In contrast, a cross-section (Fig. 4) through the same PVDF membrane
as shown in surface view in Fig. 2 reveals that surfaces 40 parallel to the plane
of the photograph are more globular and display the same cratered or pocked appearance
as the surface morphology. This more open structure in the middle portions of the
membrane is believed to be reflected as a less tortuous path of intercommunicating
flow channels in the intact membrane. This results in a decrease in the physical
length of a given flow channel from the top to the bottom of the membrane. The
flow path a liquid travels when it traverses the present membrane is therefore
much more direct than in the more tortuous channels of conventional membranes.
This is believed to be the primary reason for the increased flow rate in certain
of the present membranes as compared to conventional membranes of the same pore
size, thickness and weight of polymer.
In addition to the altered flow path length, described above, another
unique aspect of the macrostructure of the present membranes is an increased surface
area. This is particularly advantageous in applications requiring transfer of
one or more molecules to the membrane, since there is a greatly enhanced membrane
surface area at the point of contact between a molecule resting on the surface
of the PVDF membrane and the membrane itself. This is believed to result from
the greater amount of surface PVDF than in conventional membranes. Because the
surface area of PVDF polymer is high at the point of contact between a molecule
and the membrane surface, it is believed that sorption of a molecule onto the
surface of the present membrane from a target surface or another substrate will
be enhanced. The increased polymer surface area available is reflected in the
cratered or pocked macrostructural features, as illustrated in Fig. 2.
A second macrostructural morphology is illustrated by the top surface
50 of a PVDF membrane (0.10 µm pore size) shown in Fig. 5, This macrostructure
accounts for an increased surface area as compared to conventional membranes.
It is produced using a particular type of solvent, as discussed in more detail
Referring to Fig. 5, it can be clearly seen that the PVDF density
of the membrane surface is greater than that in the conventional PVDF membrane
of Fig. 1. The PVDF structure shown also lacks the interconnecting configuration
as in conventional membranes and lacks the cratered appearance of the PVDF membrane
of Fig. 2. The macrostructure illustrated in Fig. 5 is defined as being "flat"
or "flattened". The flat PVDF structure of Fig. 5 can be represented as a plurality
of small globular bodies or spheroidal nodules 52 that cover substantially the
entire surface of the membrane. These nodules can vary in size. In the embodiment
illustrated in Fig. 5, the nodules are less than about 1 micron in diameter. Many
are on the order of tenths of microns in diameter.
Figs. 6, and 7 are scanning electron micrographs of the bottom surfaces
of various PVDF membranes. In particular, Fig. 6 is the bottom surface 60 of the
conventional microporous PVDF membrane, the same membrane whose top surface is
illustrated in Fig. 1. Both top and bottom surfaces of this conventional membrane
look substantially the same. In contrast, Fig. 7 is a scanning electron micrograph
of the bottom surface 70 of the present microporous PVDF membrane previously shown
in top view (Fig. 2) and vertical cross-section (Fig. 4). The morphological features
of a cratered macrostructure 71 and globular bodies are present. Nevertheless,
these macrostructural features are less well-developed than on the top surface
(see Fig. 2). It is believed this is due to the effect of the substrate that is
in contact with the bottom surface of the membrane during formation.
One direct result of these unique morphological characteristics is
an increased flow rate of the present PVDF membranes as compared to conventional
membranes. Presented below in Table I is a comparison between the methanol bubble
points and water flow rates of the present membranes and conventional PVDF membranes
(Millipore Corp., Bedford, MA). The microporous PVDF membranes obtained by the
method of the present invention significantly differ with respect to their water
flow rate from conventional microporous PVDF membranes.
The methanol bubble points (measured in pounds per square inch above
ambient atmospheric pressure) of the present PVDF membranes are not significantly
different from the methanol bubble points of conventional PVDF membranes even
though the flow rates are higher in the present PVDF membranes. (The term "methanol
bubble point" is a well-known check on membrane performance. The bubble point
test is based on the fact that liquid is held in the intercommunicating flow channels
of microporous membranes by surface tension and that the minimum pressure required
to force liquid out of the channels is a measure of the channel diameter. Briefly,
a bubble point test is performed by prewetting the membrane with methanol on one
side, applying air pressure on the other side of the membrane and watching for
air bubbles emanating from the methanol-wetted side to indicate the passage of
air through the membrane channels. The pressure at which a steady continuous stream
of bubbles appears is the bubble point pressure). It will be appreciated that there
may be an inverse relationship between the methanol bubble point and the pore
diameter. Thus, for a given thickness of membrane, the higher the methanol bubble
point, the smaller the effective pore diameter. The fact that the methanol bubble
points are similar between the present membranes of Table I and conventional membranes
suggests that the increased flow rates of the present membranes shown in Table
I may be due to the present membranes having a greater number of flow channels
of similar pore diameter or to flow channels with less tortuous configuration
(or both) than those of conventional membranes.
COMPARISON OF PREFERRED MICROPOROUS PVDF MEMBRANE WITH CONVENTIONALLY PRODUCED
MICROPOROUS PVDF MEMBRANES
* 1 psi = 6,89 kPa
Methanol Bubble Point (psi)*
Water Flow Rate (ml/min/cm2)
Nominal Pore Size (µm)
Referring to Table I, the microporous PVDF membranes formed by the
process of the present invention: (i) encompass a wider range of pore sizes as
compared to currently available microporous PVDF membranes; (ii) have a flow rate
substantially greater than the flow rate of conventionally-made membranes having
identical thicknesses and pore sizes; and (iii) have a maximum flow rate (190 ml/min/cm2)
for a 2.0 nominal pore size that is not achieved even in a conventional microporous
PVDF membrane having more than twice the pore size.
B. The Process of Manufacture
Conventional solvent-casting processes for making microporous PVDF
membranes generally consist of: (i) applying a layer of a polyvinylidene fluoride
polymer dissolved in a solvent to a support surface; (ii) forming a film of polymer
on the support surface; and (iii) passing the surface carrying the polymer film
through a formation bath containing a nonsolvent in which the nonsolvent displaces
the solvent in the film to form the microporous membrane. The pore structure is
determined by leaching of the solvent from the film and its replacement with the
In making the initial polymer solution, conventional procedures for
making PVDF membranes often required a polymer resin to be dissolved in one solvent
at an elevated temperature (of relatively narrow range). To provide the formation
bath, a mixture of the same solvent with a nonsolvent is provided at the same temperature
as the initial polymer solution. Moreover, in forming microporous membranes using
other kinds of polymers, it has not been possible to use different initial dissolving
temperatures and formation bath temperatures in a way that can predictably form
a variety of microporous membrane products.
In contrast, the process of the present invention: (i) may utilize
a range of solvent concentrations and temperatures to predictably form porous membranes
of various pore size, polymer symmetry ratio, methanol bubble point, and water
flow rate; (ii) is not constrained to a single solvent when using PVDF; (iii) does
not require an initial PVDF polymer dissolution at substantial one elevated temperature;
and (iv) does not require a formation bath at the same temperature as the initial
Instead, the present method uses an initial solution of PVDF resin
in a solvent/co-solvent mixture. Variation of the operating parameters such as
temperature and solvent concentration gives precise and hitherto unknown control
over pore size, polymer symmetry ratio, flow rate, surface morphology and thickness
of the membrane.
Specifically, the temperature of the initial PVDF mixing solution,
the temperature of the formation bath in which the microporous membrane is formed,
and/or the relative concentration of solvents, can be selected from a range of
possible temperatures and solvent concentrations to effect a wide range of structural
and functional changes in the PVDF microporous membrane. The temperature of the
formation bath can be substantially different than the temperature of the initial
mixing solution. Specifically, one unique preferred aspect of the present invention
is that the formation bath can be effectively used at temperatures at or below
the freezing point of water (0°C).
1. Solvent Systems
Past attempts to make microporous PVDF membranes generally involve
the use of solvent materials that are volatile and are toxic. A preferred conventional
formulation solvent used to initially dissolve PVDF polymer is acetone.
See Grandine II, US-A-4,203,848. Acetone is, however, only practical for
use as a PVDF solvent at temperatures very close to its boiling point (50°C).
At temperatures much below about 50°C, it is difficult to get enough PVDF resin
into the acetone solution for practical purposes. Thus, conventional methods for
forming PVDF membranes require that PVDF resin be passed into initial solution
at temperatures near 50°C. This puts severe constraints on the final product since
the temperature of the initial solution, it has been discovered, has a most significant
effect on the final pore size and other structural features of the microporous
One unique aspect of making the present microporous PVDF membranes
is use of solutions comprising a solvent and one or more co-solvents (i.e. a "solvent:co-solvent
system"). The term "co-solvent" refers to organic solvents that dissolve PVDF resin
slowly at most temperatures. Because of this, by themselves, many co-solvents
are generally unsuitable for the methods of the present invention. They are often
unsuitable for the additional reason that they tend to break down PVDF polymer,
albeit slowly, to such an extent that the PVDF cannot reform or coagulate again
in a conventional solvent-casting process.
This property of a co-solvent, surprisingly, can be utilized in the
method of the present invention. A small amount of co-solvent added to the solvent
permits the solvent casting procedure to be carried out over a wide range of formation
bath temperatures; even including formation bath temperatures near zero degrees
According to the present invention, the co-solvents are selected from
formamide, methyl isobutyl ketone, cyclohexanone, diacetone alcohol, diisobutyl
ketone, ethylacetoacetate, triethyl phosphate, propylene carbonate, glycol ethers
and glycol ether esters. A particularly preferred co-solvent is n-butylacetate.
The term "solvent", refers to organic compounds that dissolve PVDF
rapidly at most temperatures. This term includes those compounds that can yield
a PVDF solution of at least 25% by weight. A preferred solvent of the present
invention is N-methyl-2-pyrrolidone (hereinafter "N-pyrrol") although other solvents
selected from dimethyl formamide, tetrahydrofuran methyl ethyl ketone, dimethylacetamide,
tetramethyl urea, dimethyl sulfoxide and trimethyl phosphate can be used.
The initial dissolution of PVDF is done in a solvent:co-solvent system
wherein the components are miscible with each other. In preferred embodiments,
the solvent is about 95% N-pyrrol, the balance being one or more selected co-solvents.
In preferred embodiments, at least about 50% of the formation bath
is a nonsolvent that is always miscible with the other components of the bath.
The term "non-solvent" refers to organic compounds that do not substantially dissolve
PVDF at any temperature. A portion of the same co-solvent that is in the initial
PVDF dissolution mixture also is included. The preferred nonsolvent for use in
the formation bath is methanol. Ethanol and butanol can also be used as nonsolvents,
provided that the nonsolvent liquid is miscible with the other components in the
2. Apparatus and Method
Apparatus and general methods for forming membranes according to
the present invention are well-known to those of ordinary skill in the art. For
example, techniques for making microporous membranes by hand are known. A polymer
solution is coated on a glass plate, which plate is then immersed in a formation
bath. After formation of the membrane, the glass plate is dried at an elevated
temperature and the formed membrane removed from the plate. See, US-A-3,642,668
(Bailey: issued Feb. 1972). Microporous membranes described herein were made using
this manual method.
Similarly, techniques for making PVDF membranes using a continuous
process are also well-known. See for example, US-A-4,203,848 (Grandine, II: issued
May 20, 1980). A conventional apparatus and process for continuous solvent-casting
will be briefly described.
Referring to Fig. 8, equipment for production of the present PVDF
membranes includes mixing bath 42 for holding a supply of a PVDF polymer or resin.
The mixing bath contains the substantially dry PVDF polymer a first liquid that
is a solvent for the polymer, and at least one liquid that is a co-solvent for
the polymer, as defined in claim 1 Both solvent and co-solvent are miscible with
the formation bath solution. In preferred embodiments of the invention, the first
liquid solvent of the mixing bath is preferably N-pyrrol and the co-solvent is
formamide and/or N-butylacetate.
The mixing bath is provided with apparatus such as a knife blade
44 for applying a film of the dissolved PVDF polymer solution at a substantially
uniform thickness onto a substrate 46 that is nonporous, preferably a polyester.
The substrate is carried on a roller 47. The substrate is driven through the remainder
of the apparatus by a powered take up roll 62.
During operation of the process, substrate 46 is pulled through the
system by takeup roll 62. As it travels through the system, substrate 46 passes
beneath the mixing bath, which bath includes a coating blade as a knife that is
set to a predetermined gap such as, for example, 300 µm. A film of polymer solution
is applied at a substantially uniform thickness onto the substrate that travels
from its supply roll to a position underneath the knife. The film of polymer solution
that is applied to the substrate forms with that substrate a film-substrate laminate
that is caused to travel directly into one or more formation baths.
The function of the formation bath is to convert the film to a porous
membrane. The film-substrate laminate remains immersed in the formation bath until
the pore structure is fully formed. The laminate then travels into extraction
bath 56 where much of the solvent for the polymer that may remain in a porous membrane
is displaced. It will be appreciated that, although Fig. 8 illustrates several
formation bath tanks 50 and a single extraction bath 56, the number of formation
baths will depend upon a variety of factors. Generally, more than one formation
bath is required in order to ensure that as much of the solvent as possible is
removed as possible prior to separation of the formed membrane from the substrate.
Thus, the solvent displaced from the membrane into the formation bath will gradually
enrich the formation bath. Each successive immersion in fresh formation bath liquid
will drive more of the solvent from the membrane.
A heater (not shown) heats the solvent:co-solvent solution in the
bath 42 to elevated temperatures. A mixer, also not shown, adds the polymer to
the solution. A separate system 52 is provided for selective heating and cooling
of formation bath liquid. Cooling the formation bath can be performed by any one
of a number of conventional systems, preferably provided that the cooling system
can decrease the temperature of the formation bath to at least 0°C, more preferably
to about -10°C, and increase the temperature of the formation bath to about 50°C.
The formation bath is maintained substantially in one or more tanks
50 of generally rectangular shape. The formation bath is a liquid that is always
miscible with the solvent and co-solvent but is a nonsolvent for the PVDF polymer.
The liquid is preferably methanol. The remaining components of the preferred formation
bath are compatible with the mixing bath and would include co-solvents such as
formamide and N-butylacetate in various combinations. Any solvent present in the
formation bath would be included as carry-over from the initial mixing solution.
A roller system is provided for guiding the layered substrate into,
through, and out of the adjacent formation baths. Multiple baths are provided so
that the layered substrate can move continuously from one bath to the other, the
nonsolvent in successive formation baths gradually replacing the solvent and co-solvent
from the PVDF film. Such baths have associated within them a plurality of submerged
rollers 53 designed to maintain tension of the laminate as it passes through the
formation bath. Pairs of rollers 54 are mounted adjacent to the formation baths
and after the extraction bath 56. These also apply tension to the layered substrate
as the substrate travels through this section. It would be appreciated that the
exact configuration of these tensioning rollers is not critical to the operation
of the present invention.
Extraction bath 56 is positioned adjacent to the last formation bath
and is provided with a nonsolvent, preferably water. The microporous membrane 58
spontaneously separates from the moving substrate 46 when substantially all of
the solvent has been displaced from the membrane and the membrane matrix is fully
formed. This typically occurs in the extraction bath 56.
An extraction system 60 is positioned adjacent the final extraction
bath and receives the microporous membrane 58. Extraction system 60 includes a
drum 64 provided with a suction device 66 and spray head device 68. The membrane
is disposed along an outer periphery of drum 64 in a substantially annular channel
67 formed by the drum and the inner surfaces of the suction 66 and spray head
devices 68. Suction device 66 engages the membrane against the outer periphery
of drum 64 and ensures that extraction liquid is sprayed uniformly on the membrane.
The spray head 68 applies extraction liquid, preferably water, to the membrane.
The water is applied under pressure and this helps to remove any remaining solvent
and/or co-solvent from the pores of the membrane into the extraction system. It
also removes loose particles of polymer that are on the surface of the membrane.
This is important since any loose, surficial particles may cause punctures in the
membrane if not removed at this stage.
After travelling through extraction bath 56, the membrane 58 can
be separated from the substrate 46. Although Fig. 8 illustrates separation of the
membrane after passage through a plurality of formation bath tanks, it will be
appreciated that the separation of the membrane can be achieved in a single tank
depending upon the solvent concentration and the amount of nonsolvent available
to displace the solvent. Once separation occurs, the used substrate 46 is passed
under a tension roll 70 and then onto a takeup roll 62. Membrane 58 is similarly
passed through a series of membrane tension control rolls 70 to a driven windup
Extraction system 60 provides for in-line extraction and drying.
Membrane travels over the outer surface of drum 64. Suction is applied to the surface
of the drum to constrain the membrane against shrinkage and movement. A series
of spray heads is disposed along one arcuate segment of the drum to apply extraction
liquid to the membrane and displace any solvent remaining in the membrane. When
suction is used, all or part of the sprayed liquid is drawn directly through the
membrane into the drum by the applied suction and any liquid that drips down may
be caught in a separate tray (not shown).
In the final area of the drum, heat can be applied to the membrane.
This may be accomplished by heating the drum using a device not shown such as an
infrared heater, by blowing hot air onto the membrane, or by a combination of
these and other steps. The dried membrane is then wound on a windup roll.
The final drying stage preferably involves a heat treatment. Such
a so-called "annealing" step includes heating the membrane above its expected temperature
of use to ensure that the membrane is dimensionally stable when used. The operating
temperatures of PVDF membranes of the present invention range from about 0 to about
100°C. Since the polymer has a melting point of about 160°C, a good annealing
temperature is about 130°C. The best annealing temperature is one that is closest
to the melting point of a particular grade of PVDF without actually melting the
The invention will now be described further by means of several specific
examples of preferred embodiments. In the following examples, all parts and percentages
are by weight unless otherwise specified and temperatures are in degrees Celsius.
Example 1: Production of PVDF Microporous Membranes of Various Sizes
This example illustrates the unique versatility of the present manufacturing
process and its ability to manufacture microporous PVDF membranes using control
of temperature and preselected solvent composition to produce desired membrane
The membranes were formed by hand on 6" x 8" (1" = 2,54 cm) glass
plates using conventional procedures. Briefly, an initial solution of PVDF polymer,
solvent and co-solvent were mixed and applied to the glass plate using a coating
blade having a gap width of 15 thousands of an inch (1 inch = 2,54 cm). The coated,
glass plate was immersed in a formation solution and the membrane was allowed to
form on the glass plate. Once formed, the membranes were dried at about 50°C on
the glass plates and membrane discs were produced using a 47 mm diameter punch.
The membrane thickness was measured as well as the water flow rate (ml/min/cm2)
and methanol bubble point (psi) using standard procedures.
A. Effect of Mixing Temperature and Co-Solvent Composition
An initial weight of PVDF polymer (see below) was mixed with 94-95%
N-pyrrol and 5-6% N-butylacetate at temperatures of 20°C, 35°C, 40°C, and 45°C.
The formation bath (about 20°C) contained a series of methanol (nonsolvent) and
N-pyrrol or butylacetate (co-solvent) concentrations. After the PVDF polymer was
dissolved at different initial temperatures, membranes were formed in these different
formation bath solutions. The thickness, water flow rate, and methanol bubble point
of the membranes produced under these various conditions was determined on the
extracted and dried membranes. The results for measurement of the water flow rate
are presented below in Table II for 375-398 g Kynar® 761 PVDF, (Atochem North
America, Philadelphia, PA), used at different mixing solution temperatures and
different solvent:co-solvent concentrations of the formation bath (20°C). Results
for water flow measurement are presented in Table III for 375-398 g Kynar®
761 PVDF used at different mixing solution temperatures and different solvent:co-solvent
concentrations in the formation bath (20°C).
Flow Rate (ml/min/cm2) (mean ± s.e)
a. MeOH = methanol; BA = n-butylacetate
* 1 psi = 6,89 kPa
Mixing Temperature (°C)
MeOH (100%) comparative
MeOH:BAa (90:10) invention
MeOH:N-Pyrrol (90:10) comparative
0.395 ± .007
0.515 ± 0.02
1.75 ± .007
1.75 ± 0.21
4.15 ± .007
2.3 ± .28
Thickness mean ± s.e)
160.5 ± .007
113 ± 1.4
103.5 ± 2.1
98.5 ± 3.5
Bubble Point (psi) (mean ± s.e.)*
57.5 ± .70
51.5 ± .70
37.5 ± .70
38 ± 2.8
31.5 ± .70
36 ± 1.4
B. Effect of Formation Bath Temperature and Formamide
Water Flow (ml/min/cm2)
a. MeOH = methanol; BA = n-butylacetate
Mixing Temp. (°C)
Methanol (100%) comparative
MeOH:BA:H20 (55:25:20) comparative
A series of initial mixing solutions were made up at 40°C as follows:
- Kynar®761: 400 g
- N-pyrrol: 2250 g
- n-butylacetate: 100 g
- Formamide: 100 g
- solids = 14% of total weight
- Kynar®761: 400 g
- N-pyrrol: 2350 g
- Formamide: 100 g
- solids = 14% of total weight
Membranes were formed in temperature-controlled formation baths at
0°C and 20°C. In addition, the formation bath contained a series of decreasing
methanol concentrations and increasing formamide concentrations (100% MeOH; 75:25
and 50:50 MeOH:Formamide). Membranes were formed, removed from the plate, and dried
as above and their water flow rate, thickness and methanol bubble point were measured.
Results are presented in Table IV. The dashed line indicates that a membrane did
Use of formamide as a co-solvent results in formation of a PVDF membrane
having the flattened macrostructure of Fig. 5.
RESULTS AND DISCUSSION: Operating Parameters
Water Flow (ml/min/cm2)
1 psi = 6,89 kPa
Initial mixing solution:
Formation Bath (°C)
(comparative) Methanol (100%)
75% MeOH:25% Formamide
50% MeOH:50% Formamide
Bubble Point (psi)*
From these data, some qualitative observations can be made. The co-solvent
composition of the formation bath can have a profound effect on flow rate, with
presence of co-solvent slightly lowering the bubble point, but dramatically raising
the flow rate, particularly at low formation bath temperatures (Tables II and III).
This effect of co-solvent does not extend indefinitely, however, and if concentrations
of co-solvent (especially formamide) are much greater than about 25% by weight,
the membrane either cannot form or tends to become thinner and more fragile (Table
The temperature of the initial mixture prior to formation appears
to have a great influence on membrane properties (Tables II and III). For any given
formation bath composition, the bubble point is lowered by increasing the temperature
of the initial mixing solution (Table II). This results in a dramatic increase
in flow rate as the mixing solution temperature is increased from 20°C to 45°C
Moreover, the temperature of the formation bath also has a significant
effect on the membrane properties, particularly with co-solvents such as N-butylacetate
and formamide. As the formation bath temperature increases, the resulting membrane
shows a decreased water flow rate and increased bubble point (Table IV). Cooling
the formation bath down to about 0°C while the membrane is being formed results
in the opposite effect; a significantly decreased bubble point and increased flow
rate (Table IV).
Generally, a warmer mixing solution and formation bath results in
significantly increased rate of exchange of nonsolvent for co-solvent in the pores
of the forming membrane than at lower temperatures. Further, the dissolution of
the polymer in the initial solvent is also faster at higher temperatures. This
may result in a higher number of intercommunicating flow channels and a more tortuous
flow path for liquid travel. This is reflected in the higher methanol bubble point.
In the Examples, several different values were employed for some
of the more important operating parameters. These values have been proven to produce
effective microporous membranes by many successful demonstrations. Preferred operating
ranges for some of these parameters include a PVDF polymer concentration in the
range of between about 14% to 24% by weight of the initial mixing solution at a
preferred temperature range of between about 20°C and about 50°C. The initial
solution preferably contains this weight of PVDF polymer in a mixture of 95% N-pyrrol
and 5% butylacetate. The most preferred operating conditions for production of
PVDF membranes of the present invention have a co-solvent concentration in the
formation bath no greater than about 30%. The formation bath solution is made up
at a temperature in the range of -10°C to 50°C.
Microporous membranes prepared in accordance with the present invention
that are produced in accordance herewith are particularly useful in the chemical,
food and pharmaceutical industries. One particular use in forming filters with
pore sizes on the order of about 0.1 µm or less is that the membrane may be used
for removal of viruses and large macromolecules from the fluids being processed
through the membrane. Because PVDF is chemically inert, the membrane may be steamed
Another useful industrial application of the present membranes is
in tangential or cross-flow filtration systems. In tangential or cross-flow filtration
systems, not all of the liquid volume goes through the membrane. Some fraction
of the filtrate volume can be removed and recycled. Tangential flow systems characteristically
use tube-shaped filters or cartridges where the flow is moved axially through
the tube and material is released transversely across the walls of the tube. Tangential
flow can be used at lower pressures than normal filtration and the longevity of
the filter is enhanced in this way. Because the flow rates of the present membranes
are significantly faster than those of conventional membranes of equal thickness,
tangential flow using the present membranes is particularly advantageous because
the time for processing a given volume of sample having a given particle concentration
will be much faster than processing using conventional membranes.
Furthermore, membranes can be produced according to the present invention
as a polyester supported PVDF membrane where the polyester is encapsulated as the
PVDF membrane is formed. A particularly preferred type of PVDF membrane obtainable
by the process of the present invention, however, is a PVDF fibrous support in
which the PVDF fibers are integrally formed with the PVDF polymer as the microporous
membrane is produced.