BACKGROUND OF THE INVENTION
This invention relates to high strength ultrafiltration membranes.
More particularly, this invention relates to high strength cellulosic ultrafiltration
membranes made from a microporous polymeric base-resistant substrate and a thin
cellulose or cellulose ester polymer ultrafiltration layer.
Microporous and open ultrafiltration membranes include thin sheets
and hollow fibers generally formed from polymeric material and having a substantially
continuous matrix structure containing open pores or conduits of small size. The
pore size range for pores of microporous membranes are generally understood to
extend from about 0.05 microns to about 10 microns. Composite ultrafiltration
(UF) membranes are UF membranes formed on a pre-existing microporous membrane substrate.
The composite membranes have better integrity (higher bubble points) than UF membranes
cast from the same polymer solutions onto traditional non-woven backing materials
such as a non-woven polyester substrate. For example, U. S. Patent 4,824,568 discloses
high bubble point membranes that are composites of polyvinylidene fluoride (PVDF)
or polyethersulfone solutions coated onto a 0.22 micron PVDF microporous substrate.
The PVDF solutions are based on solvents that also soften a portion of the PVDF
substrate. It is presently believed that this solvent bonding is necessary in order
to prevent delamination of the composite structure. However, EP-A-0.596.411 teaches
that the use of such a solvent system is undesirable since it can soften the microporous
substrate. The use of PVDF also is disadvantageous since PVDF is attacked by common
cleaning and sanitizing agents such as 0.5N NaOH. These PVDF based composites,
therefore, are not appropriate for use in process streams that foul membranes such
as serums, fermentation broths or other protein separation processes which then
must be cleaned and sanitized by NaOH.
At the present time, ultrafiltration membranes comprised of cellulose
are used in applications where low protein binding and low fouling characteristics
are required. Cellulose ultrafiltration membranes are formed by immersion casting
of a cellulose acetate polymer solution onto a non-woven fabric substrate formed,
for example, from polyethylene or polypropylene. The non-woven substrate has relatively
large pores, typically in the order of several hundred microns in effective diameter
in comparison to the UF layer formed on it. The UF layer is typically bound to
some degree to the substrate by mechanical interlocking of the UF layer and the
substrate. The cellulose acetate is then hydrolyzed to cellulose by using a strong
base such as 0.5N NaOH.
Alternatively, cellulose can be dissolved in solutions of solvents
such as dimethylacetamide (DMAC) or N-methyl pyrrolidone (NMP) with the addition
of a salt such as lithium chloride. This cellulose solution can be used to form
the composite membrane and subsequently eliminate the need for base hydrolysis.
While these composite membranes are considered to be generally satisfactory,
they are not considered to be defect free. A defect is an area of the membrane
where a void or rupture in the UF layer will allow passage of particles significantly
larger than the retention limit dictated by the UF layer. These defects can result
from fibers of the non-woven substrate extending though the UF layer or from gas
bubbles retained in the solution from which the cellulose acetate layer is precipitated
and coagulated which rupture the UF layer. In addition, defects are caused by the
relatively high variability of the non-woven substrate thickness which increases
the difficulty of achieving a uniformly thick UF layer. The resultant variable
UF layer thickness results in variable permeability and retention performance.
Presently available cellulosic membranes have an undesirably low
mechanical strength in that they are easily ruptured when subjected even to low
back pressure or when folded even to a minor degree. Delamination of presently
available cellulosic membranes is commonly observed at low back pressures of 0.2
to 1.0 bar (about 3 to 15 psi). Since such membranes can be exposed to some back-pressure
during use, resistance to delamination under such back-pressure conditions is
Accordingly, it would be desirable to provide a cellulosic ultrafiltration
membrane which is free of defects, exhibits low protein binding, is stable in high
pH solutions, is highly resistant to high back pressure and is mechanically stable
even when folded. Such a composite membrane could be highly useful for processing
protein-containing solutions under conditions of repeated use. It would also maintain
membrane integrity while under conditions of pressure normally encountered during
SUMMARY OF THE INVENTION
The present invention provides a defect free cellulosic ultrafiltration
membrane formed from a microporous polymeric substrate which is resistant to high
pH (base) conditions and a cellulose ester or cellulose ultrafiltration layer.
The thickness of the ultrafiltration layer is controlled so that it is rendered
defect free. The degree of penetration of the cellulose ester or cellulose into
the base resistant microporous polymeric layer is controlled so as to prevent excessive
plugging of the substrate pores thereby to maintain desirable flux characteristics
for the cellulosic membrane. The degree of penetration of the cellulose ester or
cellulose into the base resistant polymeric microporous layer is also controlled
to obtain a sufficiently strong bond between the two layers. This renders the cellulosic
membrane highly resistant to delamination.
The cellulosic ultrafiltration membrane of this invention can be formed
by passing a base resistant polymeric microporous substrate and a solution of cellulose
or a cellulose ester into a nip formed by (a) a rubber roll with or without a film
thereon and (b) a rotating cylinder. The thickness of the ultrafiltration layer
and the degree of penetration of the ultrafiltration layer into the microporous
layer is controlled by the pressure at the nip, the durometer (hardness) and diameter
of the rubber roll, solution viscosity and process speed. Optionally, the cellulose
or cellulose ester solution can be applied to the base resistant microporous polymeric
substrate by a conventional knife-over-roll or slot die coating methods. The coated
microporous substrate is then contacted with a non-solvent for the cellulose or
cellulose ester to effect its precipitation to form the ultrafiltration layer.
The cellulose ester can be converted to cellulose by reaction with a base such
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 illustrates an apparatus suitable for forming the composite
ultrafiltration membrane of this invention.
Fig. 1A illustrates an alternative apparatus suitable for forming
the composite ultrafiltration membrane of this invention.
Fig. 1B illustrates an alternative apparatus suitable for forming
the composite ultrafiltration membrane of this invention.
Fig. 2 is a photomicrograph at 396 x magnification of the cross section
of the membrane typical of this invention.
Fig. 3 is a photomicrograph at 1994 x magnification of the cross section
of a membrane of Fig. 2 showing the cellulose ultrafiltration layer and its interlocking
with the microporous ultra-high-molecular-weight polyethylene substrate.
Fig. 4 is a photomicrograph at 396 x magnification of a membrane of
Fig. 5 is a photomicrograph at 1060 x magnification of the membrane
of Fig. 4.
Fig. 6 is a photomicrograph at 397 x magnification of the membrane
of this invention.
Fig. 7 is a photomicrograph at 1002 x magnification of the membrane
of Fig. 6.
DESCRIPTION OF SPECIFIC EMBODIMENTS
The defect-free cellulosic membrane of this invention comprises a
nonfibrous base resistant polymeric microporous substrate coated with an ultrafiltration
layer formed of a cellulose or cellulose ester polymer. A major advantage of the
cellulosic membranes of this invention is that they are resistant to degradation
by contact with strong alkali solution, such as 0.5N NaOH. The composite membranes
of this invention are also resistant to back pressures far exceeding those of the
prior art and usually as high as about 100 psi (6.9 bar) . The bond between the
substrate and the ultrafiltration layer can be made sufficiently strong as to exceed
the burst strength of the overall membrane. In addition, since the cellulosic ultrafiltration
membrane is defect free, the integrity of the ultrafiltration membrane of the invention
is substantially superior to the integrity of prior-art composite membranes.
The base resistant microporous polymeric membrane has pore sizes between
about 0.05 and 10 microns, preferably between about 0.2 and 1.0 microns. Suitable
base resistant microporous membrane substrates are formed from a polyolefin such
as polyethylene or polypropylene; polysulfone, polyethersulfone, polyarylsulfone,
polytetrafluoroethylene, cellulose or the like. Particularly suitable microporous
membrane substrates are formed from ultrahigh molecular weight polyethylene (UHMW-PE)
such as those disclosed by U.S. Patents 4,828,772 and 4, 778, 601. A particularly
suitable microporous polypropylene membrane substrate is disclosed by U.S. Patent
4,874,567. The base resistant microporous polymeric membrane substrate is not
degraded when contacted with a base solution utilized to convert cellulose ester
The ultrafiltration layer is formed from a solution of cellulose or
a cellulose ester such as cellulose diacetate, cellulose triacetate, cellulose
nitrate or mixtures thereof. After being deposited on the microporous membrane
substrate the cellulose ester can be converted to cellulose by reaction with an
aqueous basic solution such as NaOH, KOH, LiOH at a pH between about 11.8 and 12.2.
The cellulose ester solution is formed with a solvent composition such as acetone,
N-methyl pyrrolidone (NMP), dimethylacetamide (DMAC), mixtures thereof or the like.
Coating techniques useful for forming the composite ultrafiltration
membrane are disclosed for example by U. S. Patents 5, 017, 292 and 5, 096, 637.
Polymer solutions containing between about 8 and 25 % by weight of
the cellulose ester or cellulose polymer in a solvent can be utilized in the present
invention. Such solutions can be coated to a dry thickness above the microporous
substrate of from 1 microns to 20 microns, preferably from 5 to 15 microns. Controlling
the coating thickness within these limits promotes penetration of the coating into
the substrate over a distance between 5 and 30 microns, preferably between 15 and
25 microns. This results in a cellulosic membrane which can be folded or subjected
to moderate or high back pressures without rupturing the cellulosic membrane.
After the cellulose ester polymer solution has been cast onto the
microporous membrane, the ultrafiltration membrane structure is formed by immersing
the coated microporous structure into a liquid which is miscible with the polymer
solvent but is a non solvent for the dissolved cellulose or cellulose ester polymer.
Water is the preferred liquid, although other liquids can be employed such as water-alcohol,
water-polymer solvent, water-glycerin mixtures.
Membrane formation occurs by precipitation of the polymer from the
polymer solution. The precipitated polymer forms a porous membrane which may or
may not be "skinned" or have an asymmetric structure typical of some ultrafiltration
membranes but which is substantially defect free. The properties of the membrane
can be varied by controlling such parameters as the percent polymer in solution,
solvent type, additives, coating thickness, immersion bath composition, immersion
bath temperature, etc.
In an aspect of this invention, a hydrophobic microporous base resistant
membrane substrate can be modified to render it hydrophilic prior to applying the
precursor polymeric solution which forms the ultrafiltration membrane layer. An
example of a suitable process is disclosed in U. S. Patent 4,618,553. A membrane
having a completely hydrophilic surface is particularly useful when filtering aqueous
Referring to Fig. 1, coating thickness is controlled by forming a
nip between a rotating drum 10 and a non-rotating rubber coated cylinder 12. The
microporous substrate 14 can be positioned on an optional support web 18 which
contacts the backed drum or roll 10 which can be rotating. The cast polymer solution
28 forms the skin in the final composite membrane of this invention.
In operation, cellulose or a cellulose ester casting solution 20 is
fed to a reservoir on the web entry side of the nip point 26 of the rubber covered
cylinder 12 and the drum 10. The moving microporous substrate 14 drags solution
under the nip 26 analogous to journal bearing lubrication. Coating thickness can
then be varied by adjusting the inlet pressure to the pneumatic cylinders 32.
Referring to Fig. 1A, coating thickness is controlled in an alternative
apparatus by changing the width of the slot 34, the delivery rate of the polymer
solution 30 which is fed to the slot die 12 by means of a suitable positive displacement
pump such as a variable speed precision gear pump or the substrate speed. In practice,
solution viscosity and casting speed are set primarily by membrane property requirements.
The width of the gap 34 is sized based on polymer solution viscosity and thickness
of coating (volume) required. The equalization chamber 32 is used to uniformly
distribute polymer solution along the entire width of the gap 34.
Referring to Fig. 1B, coating thickness is controlled in a second
alternative apparatus by changing the gap 34 between the knife 12 and the substrate
14 to be coated. The coating thickness is also affected by the reservoir height
and substrate speed. In practice, the polymer solution is fed to reservoir 30 at
a rate which will maintain the solution level in the reservoir constant and the
casting (substrate) speed is set primarily by membrane property requirements.
After the membrane structure has formed, the composite web is washed
by conveying the coated and precipitated web through a water bath. Contact time
of approximately 2 minutes in 25° C water, for example, is sufficient. When initially
coating with a cellulose ester solution, the cellulose ester UF composite is then
treated with NaOH at high pH to hydrolyze the UF layer. The regenerated cellulose
UF composite is then soaked in a humectant solution such as glycerine and water.
Drying can then be performed by leaving the rewashed web to dry as single sheets
at room temperature. Alternatively, the web can be continuously dried by conveying
the web over a heated roll.
The following examples illustrate the present invention.
This example illustrates a process making a 15 kD nominal molecular
weight cutoff composite ultrafiltration membrane.
Ultrahigh-molecular-weight polyethylene microporous membrane produced
by the process of U.S. Patents 4,778,601 and 4,828,772 having an average pore size
of 0.3-0.4 micrometers were employed as the microporous membrane substrate.
A polymer solution containing 20.0 wt% cellulose acetate in 80.0 wt%
N-methyl pyrrolidone (NMP) (solvent) was cast onto the microporous polyethylene
membrane at a speed of 3.05 m/min (10 feet per minute) utilizing the applicator
of Fig. 1 with 3.1 bar (45 psi) applied pressure. The NMP solvent is not a solvent
for the polyethylene substrate. Thus, there is no adhesion between the polyethylene
and the cellulose acetate due to solvent bonding. The coated membrane was then
immersed in a water bath maintained at a temperature of 30° C. The composite membrane
thus produced was subsequently immersed in 0.5N NaOH at 20 °C for 4 hours in order
to convert the cellulose acetate to cellulose by hydrolysis. The composite membrane
then was washed in water and treated with 20 vol% glycerine in water solution to
act as a humectant. It was subsequently dried by conveying the web over a drying
roll heated to about 54°C (130° F).
The cross-section of the composite membrane produced is shown in Figs
2 and 3. The depth of cellulose infiltration into this polyethylene substrate was
approximately 10 microns and the depth of the polyethylene substrate free of cellulose
was about 135 microns. The thickness of the cellulose coating above the surface
to the UHMW-PE substrate was about 11 microns. The composite membrane was free
of defects and had the flux and retention characteristics listed in Table 1.
This example illustrates a process making a 110 kD nominal molecular
weight cutoff composite ultrafiltration membrane. The method used for Example 2
is the same as in Example 1 with the following exceptions: the polymer solution
used was composed of 12.0 wt% cellulose acetate in 88.0 wt% N-methyl pyrrolidone
(NMP), the coated membrane was immersed in a water bath maintained at a temperature
of 10° C . All other process steps were the same.
The cross-section of the composite membrane produced by Example 2
is shown in Figs 4 and 5. The depth of cellulose infiltration into this polyethylene
substrate was approximately 25 microns and the depth of the polyethylene substrate
free of cellulose was about 130 microns. The thickness of the cellulose coating
above the surface to the UHMW-PE substrate was about 3 microns. The composite
membrane was free of defects and had flux and retention characteristics listed in
This example illustrates a process making a 1,300 kD nominal molecular
weight cutoff composite ultrafiltration membrane. The method used for Example 3
is the same as in Example 2 with the following exceptions: the coated membrane
was immersed in a water bath maintained at a temperature of 50° C.
The cross-section of the composite membrane produced by Example 3
is shown in Figs 6 and 7. The depth of cellulose infiltration into this polyethylene
substrate was approximately 35 microns and the depth of the polyethylene substrate
free of cellulose was about 130 microns. The thickness of the cellulose coating
above the surface to the UHMW-PE substrate was about 12 microns. Although this
membrane clearly contained macrovoids, it was still capable of withstanding high
backpressure and had flux and retention characteristics listed in Table 1.
Water Flux, 90% Dextran Rejection, Backpressure Capability for Some Cellulose
Composite Membranes versus Conventional Membranes
** Backpressure test system limitation is 4.1 bar (60 psig).
***PLAC, PLGC, PLTK, PLHK and PLMK are conventional cellulose
ulrafiltration membranes manufactured by Millipore Corporation which utilize a Freudenberg
non-woven substrate. Values indicated are typical of these membranes.
90% Dextran Rejection kD
Flux, gfd/psi (see Note)
Water Visual Bubble Point, bar (psig)
Reverse Pressure Failure, bar (psig)
Note: In the third column in Table 1, the figures are litres/m2/day/bar,
and the corresponding figures in parentheses are in gfd/psi.
This test is carried out using a computer controlled automated test
system. This system collects data on weight of permeate passing through a test
cell as a function of elapsed time. The automated system controls pressure to the
inlet of the test cell and temperature of the water. The test pressure for the
membranes described was 1.7 bar (25 psig) and the water temperature was 25 Degrees
Celsius. A minimum of 15 grams (ml) of water was collected for each membrane tested.
The flux was calculated using the membrane area in the test cell open to the flow
path, the volume collected, elapsed time and test pressure.
90% DEXTRAN REJECTION
Rejection of model solutes is the most common method for describing
the expected performance of ultrafiltration membranes. Thus, nominal molecular
weight limits (NMWL) can be determined with a variety of solutes; frequently proteins
are used. The NMWL of a UF membrane is typically the molecular mass of the smallest
protein that the membrane rejects at a chosen level, usually 90 to 95%. Other solutes
that can be used to characterize UF membranes include dextrans, which are available
in a large range of molecular weights. The whole rejection spectrum, from molecules
of about 1000 daltons molecular weights to molecules of about 2,000,00 daltons
to can be measured in a single test.
The test is based on methods published, inter alia, by L. Zeman and
M. Wales, in "Separation Science and Technology" 16 (30), p. 275-290 (1981). The
membranes to be characterized are challenged with solutions containing polydisperse
dextrans with molecular weights 1000 to 2,000,00 daltons in a suitable device;
the permeation rate during the test is controlled at low flux to minimize concentration
polarization. Feed and permeate streams are sampled and analyzed by size exclusion
chromatography (SEC); the chromatographic data is used to calculate rejection as
a function of dextran molecular mass.
Rejection (R) with dextran molecular mass is R=1 - Cp/Cf, where Cp
and Cf are the dextran concentrations of given molecular mass in the feed and the
permeate, respectively. The molecular weight at which the membrane retains 90%
of the dextran feed is the 90% dextran rejection value. Although it is common to
call the 90% dextran rejection value a cutoff, care must be taken to distinguish
it from cutoffs measured with other solutes such as the solutes used to determine
VISUAL BUBBLE POINT
The visual bubble point test is used to determine the maximum pore
size (or defect) of a permeable membrane. The test is based on the fact that liquid
is held in the membrane pores by surface tension effects. The minimum pressure
required to force liquid out of the pore is a measure of pore diameter as described
by the Washburn equation:
P = k 4 δ cos &phis; / d
ASTM Method 316-80 is employed. In general, this bubble point test is performed
by prewetting the filter with the liquid to be used (water for the purposes of
this work), increasing the pressure of air up-stream of the filter and watching
for bubbles downstream of the filter. The pressure at which a continuous stream
of bubbles appears is the visual bubble point.
- P = bubble point pressure
- δ = surface tension
- &phis; = liquid/solid contact angle
- d = pore diameter
- k = shape correction factor
This test is conducted with a system consisting of a modified 293mm
membrane holder (Millipore part number YY3029316), a regulated air pressure source,
rotameters to measure air flow (up to 72 cc/min) and miscellaneous valves and tubing.
A water wetted membrane is first placed in the test cell. A plastic screen with
a 3 mm x 3 mm pattern 0.5mm thick is placed on top of the membrane's ultrafiltration
layer. The cell is then closed and sealed. Pressure is applied to the substrate
side of the membrane while monitoring both the flow of air upstream from the membrane
with rotameters and bubbles formed by the displacement of air downstream of the
membrane from a tube inserted into a water filled beaker and connected at the other
end to the test cell discharge port. The screen placed in the cell with the membrane
allows the ultrafiltration layer to delaminate under the stress of the pressure
applied to the substrate side of the membrane. When this occurs, a sharp increase
in the flow of air is detected and recorded. The system is designed with a 4.1
bar (60 psig) maximum pressure capability.