This invention relates to a polyvinylidene difluoride (PVDF) membrane.
More particularly, it relates to microporous PVDF membranes which are capable
of conversion to a hydrophilic form and which, when converted, have substantially
uniform hydrophilic properties.
Microporous membranes useful as filter materials are frequently made
from materials which are thermoplastic polymers, an example of which is PVDF.
Membranes made from PVDF are generally chemically inert and, as such, are useful
for filtration of a wide variety of fluids. However, membranes made from PVDF
are not inherently wettable by water. The natural hydrophobicity of PVDF membranes
limits their usefulness in the filtration of aqueous solutions. In practice, this
limitation is overcome by treating the PVDF membrane by a process which modifies
its exposed surfaces to make it hydrophilic.
Many such processes have been described in the art. In U. S. Patent
4,774,132, a PVDF membrane is first treated with strong alkali following which
a polymer of acrylic acid is grafted onto its surface. In U. S. Patent 4,341,615,
a crosslinked polymer formed from acrylic acid, a multiply unsaturated crosslinking
agent and a free-radical polymerization initiator is formed in situ in the
porous structure, thereby imparting hydrophilicity to the membrane. Other processes
for rendering a PVDF membrane hydrophilic are also known to those skilled in the
art of membrane and filtration technology.
Different wetting properties are required for different applications
of membranes. Some applications require wettability by liquids of extremely high
surface tensions, while others only require wettability by water. Such different
degrees of wettability can be imparted to hydrophobic PVDF membranes by selection
of an appropriate process such as one of those cited above. In order to be useful
as a filtration material, however, the wetting properties of the membrane should
be as uniform as possible and the membrane should also be as mechanically sound
as possible. While the processes known to make PVDF membranes hydrophilic are
generally successful, the product is not always uniform with respect to its wetting
and mechanical properties. This is generally a reflection of the non-uniform nature
of the starting material, i.e., the hydrophobic PVDF membrane.
The present invention provides for a method for enhancing the characteristics
of a hydrophobic polyvinylidene difluoride membrane comprising heating the membrane
to a temperature of at least about 80°C but less than the temperature at which
the membrane softens and deforms for a time sufficient to achieve a state such
that, when subsequently hydrophilized, the resultant hydrophilic membrane has
substantially uniform hydrophilic properties.
The present invention also provides for a method for preparing a
microporous, hydrophilic, polyvinylidene difluoride membrane with enhanced characteristics
comprising (1) heating the membrane in a hydrophobic form to a temperature of at
least about 80°C but less than the temperature at which the membrane softens and
deforms for a time sufficient to achieve a state such that, when subsequently
hydrophilized, the resultant hydrophilic membrane has substantially uniform hydrophilic
properties, and (2) thereafter hydrophilizing the heat treated hydrophobic membrane.
The present invention further provides for a heat-treated, hydrophobic,
polyvinylidene difluoride membrane having uniform crystallinity as evidenced by
a difference in crystallinity over the membrane of less than about 10 percent.
The present invention also provides for a hydrophilic, polyvinylidene
difluoride membrane prepared from a heat-treated, hydrophobic, polyvinylidene
difluoride membrane which remains hydrophilic after heating to a temperature as
high as about 121°C.
The present invention overcomes the difficulties described above.
More specifically, the present invention provides for a PVDF membrane having more
uniform characteristics in order to be able to prepare a hydrophilic PVDF membrane
with uniform wetting and mechanical properties. The present invention further
provides a method for making a PVDF membrane having more uniform physical properties.
The present invention also provides for a means for making a hydrophilic thermoplastic
polymer membrane having uniform wetting and mechanical properties which remain
hydrophilic even after being heated to a temperature as high as about 121°C.
Membranes made from thermoplastic polymers, such as PVDF membranes,
are usually made by phase-inversion processes wherein the precipitation of polymer
in the form of a membrane is either thermally or solvent-induced from a polymer
solution. After the membrane is formed, it is typically washed and dried before
being processed further into a hydrophilic filter material. The economics of membrane
manufacture dictate that the membrane be dried in continuous lengths as rapidly
Surprisingly, it has been found that the wetting properties of the
resultant hydrophilic PVDF membrane depend on the thermal history of the hydrophobic
PVDF membrane from which it was made. A hydrophilic membrane made from a conventional
hydrophobic PVDF membrane which had previously been heated for some time at an
elevated temperature, e.g., at about 100°C, has a higher critical surface wetting
tension (CWST) than a hydrophilic membrane made from a similar, but unheated, hydrophobic
PVDF membrane. (The term "critical wetting surface tension" as used herein refers
to the wetting characteristics for a porous media as defined in detail in U. S.
Patent 4,923,620. Basically, it is an indication of the ability of a porous medium
to absorb or be wetted by a liquid with a specified surface tension.) Additionally,
if the membrane had been subjected to potentially degrading chemical reactions
during the hydrophilization process, the mechanical properties of the PVDF substrate
are better retained after the hydrophilization process than if the PVDF membrane
had not been first heated in such a manner. Furthermore, it has been found that
when a PVDF membrane is hydrophilized after having been heated as described above,
its wetting properties are more uniform and superior to those of a similar membrane
which had not first been heated but had been hydrophilized by the same method.
While the effect of heat on the PVDF membrane is not completely understood,
it is believed that heating the hydrophobic membrane before hydrophilization alters
the crystallinity of the polymer making up the membrane and brings substantially
all of the polymer in the membrane to the same state of crystallinity. It is further
believed that hydrophilization of a more highly crystalline membrane leads to
a hydrophilic membrane having a higher CWST and greater mechanical strength than
does similar treatment of a less crystalline membrane.
Despite the use of careful controls during membrane drying processes,
even though the membrane emerges from these processes in the dry state, i.e.,
there is no residual moisture in the membrane, the crystallinity of the resultant
dry membrane is not uniform. Depending on the drying process used, it is often
observed that there are irregularly-shaped areas in the membrane sheet which do
not have the same crystallinity as that of the bulk of the membrane. Further,
sometimes one side of the membrane does not have the same crystallinity as the
other side, or other non-uniformities in crystallinity are observed.
Such non-uniformity is undesirable because after the membrane is
hydrophilized these areas of differing crystallinity appear as areas which have
different CWST, color, or mechanical properties from that of the bulk of the membrane.
For example, they may appear as hydrophobic areas in a generally hydrophilic membrane
sheet, as discolored areas, or as weak spots. The subject invention provides for
bringing the dry PVDF membrane to a state of uniform characteristics, thereby
enabling the production of a hydrophilic membrane having uniform wetting and mechanical
The present invention provides for a dry, thermoplastic, microporous
PVDF membrane, typically having a pore rating in the range of from 0.01 to 1.0
micrometer, to be heated to a temperature of about 80°C or higher for a time sufficient
to achieve a state such that, when subsequently hydrophilized, the resultant hydrophilized
membrane has substantially uniform hydrophilic properties. By "substantially uniform
hydrophilic properties" is meant that the membrane exhibits substantially the
same response when contacted with a liquid regardless of the position on a given
surface of the membrane. In general, the only upper restriction on temperature
is that the temperature must not be so high that the membrane becomes soft and
deforms, either under its own weight or due to tension from any mechanical means
by which the membrane is supported during the heating process. Typically, this
upper temperature limitation will be about 160°C.
The higher the heating temperature, the shorter the period of time
required to ensure uniformity of the membrane. The time required for the process
depends on the heating method used and on the bulk form of the membrane while
it is heated. Small pieces of membrane in flat sheet form may require only a few
minutes, e.g., five minutes, exposure to heat because of the large exposed area
available for transfer of heat to the membrane. On the other hand, a membrane
which is rolled up tightly in a roll containing hundreds of linear feet (1 foot
= 0.3048 meter) of material may require many hours for all the membrane to come
to an equilibrium temperature.
Any means for heating the membrane may be employed. However, the
source of heat must be controlled so that no portion of the membrane exceeds the
temperature at which deformation occurs, typically about 160°C. Further, any mechanical
means used to support the membrane during the heating process must not damage
the membrane due to pressure, tension, or other physical contact with the membrane.
It has been found convenient to make PVDF membrane having uniform
properties by heating rolls containing from 500 to 1000 linear feet (1 foot =
0.3048 meter) of dry membrane in a circulating air oven. In general, when a roll
of membrane is heated in this manner, a minimum period of about sixteen hours
is required for all sections of the roll of membrane to become heated to the same
temperature. After sufficient time for all sections of the membrane to come to
the same temperature and then for the crystallinity of all areas of the membrane
to reach the same level, the resultant membrane can be hydrophilized by known means
to yield a membrane with stable, uniform wetting and mechanical properties.
When heating a roll of PVDF membrane in an oven, temperatures ranging
from 80°C to 160°C may be used. When a temperature of 80°C is used, the time required
to achieve uniformity is about sixty-four hours. When the heating temperature is
120°C or higher, a period of about sixteen hours is generally sufficient. Heating
temperatures ranging from 80° to 145°C are preferred. Temperatures ranging from
100°C to 120°C are most preferred. In this temperature range, uniformity is achieved
in a practical period of time and no change in structure, pore size, or overall
dimensions of the membrane is observed.
The minimum degree of crystallinity which is required depends on
the method which will be used to make the thermoplastic membrane uniformly hydrophilic.
This can be determined easily by evaluating hydrophilic membranes made from heated
membranes according to means known to those skilled in the art. However, in general,
the higher the degree of crystallinity, the more uniform the membrane is and the
higher the CWST which can be achieved by any given means of hydrophilization.
Furthermore, the higher the degree of crystallinity of the PVDF membrane, the more
stable the hydrophilic surface will be when exposed to heat. This is useful because
such membranes can be exposed to temperatures such as those seen in autoclave or
dry heat sterilization cycles, e.g., at about 121°C, without the membrane losing
its hydrophilic character.
Typically, a PVDF membrane, after heat treatment as provided by this
invention, will demonstrate differences in crystallinity over the membrane of
less than about 10 percent and preferably less than about 5 percent. Crystallinity
is typically determined by X-ray crystallography or differential scanning colorimetry.
Area of Industrial Applicability
Membranes as provided by the present invention after conversion to
a hydrophilic form have uniform hydrophilicity, high mechanical strength, high
CWST, and retain their hydrophilic characteristics after exposure to heat, for
example, during autoclaving. They are useful in many filtration applications and
particularly useful in the food-processing and pharmaceutical industries where
sanitization or sterilization by means of heat is required.