TITLE: Method for preparing a membrane of a film-forming non-sulfonated
sulfone polymer.
European Patent Application EP0036947 A1

ABSTRACT:
Sulfone polymer ultrafiltration membranes with remarkably high
permeability to solvent at low pressure combined with excellent
retention of low molecular weight macro-molecules and sharp molecular
weight cutoff are prepared by casting a sheet of said polymer dissolved
in a specified solvent (hexamethylphosphoramide) evaporating a portion
of the solvent from the sheet and immersing said sheet in a gelation
liquid therefore.

INVENTORS:
Benzinger, William Donald
Hinde, George Morralee
Toal, Margaret Gavin

APPLICATION NUMBER: EP19810101354
PUBLICATION DATE: 10/07/1981
FILING DATE: 02/25/1981

ASSIGNEE: PENNWALT CORP (US)
INTERNATIONAL CLASSES: C08G75/00; B01D71/68; C08J3/09; C08J9/28; C08L81/06; (IPC1-7):
B01D13/04; C08L81/06
EUROPEAN CLASSES: B01D71/68; B01D61/14D; B01D67/00K14B; C08J3/09B6+L81/06

FOREIGN REFERENCES:
GB2025312A

CLAIMS:
CLAIMS
1. A membrane of a film-forming, non-sulfonated sulfone polymer having
the following characteristics when used as a fi?. t:ration means: a) a
water flux of at least 0.2 ml./cm.2/min. at 30 psig. b) a retention of
at least 95% of polyethylene glycol having an average molecular weight
in the range of about 15,000 to about 20,000, c) a retention of at
least 808 of polyethylene glycol having an average molecular weight of
about 6000, and d) a retention of less than 10 of polyethylene glycol
having an average molecular weight of about 1000.
2. The membrane of claim 1 wherein said water flux is 2 at least 1.0
ml./cm /min.
3. The membrane of claim 2 wherein said retention of polyethylene
glycol having an average molecular weight in the range of about 15,000
to about 20,000 is at least 99%, and said retention of polyethylene
glycol having an average molecular weight of about 6000 is at least
908.
4. The membrane of claim 1, 2 or 3 wherein the sulfone polymer is one
having repeating units of the following structure: EMI17.1 where n is
an integer of sufficient length to provide a filmforming polymer.
5. A method of preparing a membrane of a film-forming, non-sulfonated
sulfone polymer comprising diss#ving said sulfone polymer in hexamethyl
phosphoramide to form a solution containing from about 10 to about 25
weight percent polymer, casting said solution on a smooth solid surface
to form a film, evaporating a portion of the solvent from the film, and
then immersing said film in a gelation medium therefor at a temperature
ranging from about -100C to about 500C for a period of from about 1 to
about 30 minutes to form said membrane.
6. The process of claim 5 wherein the solution contains from about 14
to about 18 weight percent of said sulfone polymer.
7. The proce#ss of claims 5 or 6 wherein said sulfone polymer is one
having repeating units of the following structure: EMI17.2 where n is
an integer of sufficient length to provide a filmforming polymer.
8. The process of claim 7 wherein said solution contains from about 14
to about 18 percent of said polymer.

DESCRIPTION:
Porous Sulfone Polymer Membrane and Process for Its Preparation
Background of the Invention The Problem Menibranes to be used for
pressure-activated separation processes such as ultrafiltration and
reverse osmosis should have high flux, high retention of materials with
large molecular or particle size, and sharp molecular size cut-off. In
other words, molecules of solvent and of materials of smaller molecular
size than the size desired to be retained should pass rapidly and
completely through the membrane at relatively low applied pressure,
whereas molecules or particles of size equal to or larger than the size
desired to be retained should not be able to enter the pores of the
membrane.
In practice, no membrane has all pores of exactly the same size, and
therefore there is always a range of molecular sizes that are partially
but not totally retained by the membrane.
The smaller this range of sizes partially retained by the membrane is,
the sharper the molecular size cut-off of the m#xr###rane is said to
be.
Because the size of molecules are difficult to measure, it is generally
assumed that the sizes of molecules of similar types are proportional
to the molecular weight of the molecules.
Hence, instead of being characterized by the size of the molecules it
will retain, a membrane is characterized by the molecular weight of the
molecule it will retain. In like manner, the molecular weight cut-off
of the membrane is a measure of the molecular size cut-off, and hence,
the pore size of the membrane.
In addition to the above mentioned properties a membrane must have good
long term resistance to the thermal and chemical environment that it
will encounter in service. Because many commercial ultrafiltration
processes are best operated at temperatures up to about 1000C and
involve processing of acidic or basic materials, the membrane should
withstand such temperaturcs and be stable over a wide pH range. The
thermal and chemical stability of a membrane is principally determined
by the chemical nature of the polymer used to form the membrane The
class of polymers known as polysulfones and/or polyether/ sulfones have
been shown to be advantageous for forming chemically and thermally
resistant ultrafiltration membranes.
One example of such polysulfone polymer is a material sold under the
trade name Udel i 1700 by Union Carbide corporation. The chemical
structure of this polymer is EMI3.1 Methods for casting asymmetric
membranes from this class of polymers have been disclosed for example
in U. S. Patents 3,567,810, 3,615,024, 3,632,404, 3,651,030, 3,691,068,
3,709,841, 3,855,122, 3,912,834, 4,005,012, 4,026,977, and 4,038,351.
The general procedure consists of forming a solution of the polymer in
a suitable solvent or mixture of solvents, sometimes with the addition
of non-solvents or flux-promoting agents, casting a film of the
solution, evaporating a portion of the solvent, and gelation of the
membrane by immersion of the film in a liquid that is a non-solvent for
the polymer.
The properties of the resulting membrane, such as flux and retention
characteristics, depend primarily on the choice of solvent and other
additives in the casting solution and on the conditions employed during
the evaporation and gelation procedures. For example in U.S. 3,567,810,
Baker discloses that the properties of the membrane are improved when
the film is exposed to hot gas or air for 5 to 60 seconds during the
evaporation step prior to gelation.
In the above mentioned patents, a number of solvents such as dimethyl
formamide, dimethyl acetalsde, dimethyl sulfoxide, N-methyl
pyrrolidone, methyl cellosolve, and mixtures thereof are disclosed. In
U. S. 3,855,122 hexamethyl phosphoramide is mentioned as a solvent for
sulfonated polyaryl-ether/sulfone, but no example of its use is given.
Although some of the polysulfone membranes disclosed in the
above-mentioned patents have high water permeability, none of them have
good retention of lower molecular weight macromolecules such as
macromolecules with molecular weights of about 20,000 or less and none
demonstrate sharp molecular weight cut-offs In general when the pore
size of a membrane is decreased so that it retains lower molecular
weight specieS the water permeability also decreases. Thus the better
the retention of the membrane becomes for lower molecular weight
molecules, the lower the water permeability becomes.
Statement of the Invention This invention relates to a membrane of a
film-forming, non-sulfonated sulfone polymer having the following
characteristics when used as a filtration means: a) a water flux of at
least 0.2milliliters per square centimeter per minute at 30 psig, b) a
retention of at least 95% of polyethylene glycol (PEG) having an
average molecular weight in the range of about 15000 and about 20000,
c) a retention of at least 80% of polyethylene glycol having an average
molecular weight of about 6000, and d) a retention of less than 10% of
polyethylene glycol having an average molecular weight of about 1000.
The water flux of the membrane is preferably at least 1.0 ml./cm /min.,
the retention of PEG(MW 15000-20000) is preferably at least 99% and the
retention of PEG (MW 6000) is preferably at least 90%.
This invention also relates to the process of preparing the above
characterized sulfone polymer membrane which comprises dissolving a
film-forming, non-sulfonated sulfone polymer in hexamethyl
phosphoramide to form a solution containing from about 10 to about 25,
preferably about 14 to 18, weight percent polymer, casting said
solution on a solid surface to form a film, evaporating a portion of
the solvent, and then immersing said film in a gelation medium such as
water at a temperature of from -10 C to + 500C for a period of from
about 1 minute to about 30 min. to form the membrane.
Optionally, the gelled membrane may subsequently be dimensionally
stabilized by heat treatment thereof.
The preparation of the porous membranes by the process of this
invention and the ultrafiltration properties of the resultant membranes
are demonstrated by the following examples.
Example 1 A solution containing 15% polysulfone resin in hexamethyl
phosphoramide was prepared by stirring a mixture of polysulfone resin
pellets (Udel P-1700, Union Carbide Corp.) at about 95tC for about 4
hours until all of the polymer had dissolved. The solution was allowed
to stand undisturbed for about 20 min.
to allow air bubbles to rise to the surface and escape. The resulting
solution was clear and bubble free.
A 10-mil film of this solution was cast onto a smooth, clean glass
plate. Tape had been placed along the edges of the plate prior to
casting to assure adherence of the film to the plate during subsequent
processing steps. Iwmediately after the film had been cast, the plate
and cast film were placed under an electric heater at a temperature of
880C for 1 min. The plate and film were then removed and immersed in a
water bath at ambient temperature for gelation. After 30 min. the
resulting membrane was cut free of the tape, removed from the glass
plate, and stored in water at ambient temperature.
The above produced membrane was evaluated for flux and retention at 30
psig using distilled water, Blue Dextran 2000 solution, and
polyethylene glycol ("Carbowax") solutions.
Data are shown in Table 1.
Example 2 Membrane prepared as in Example 1 except that the polysulfone
concentration in the casting solution was 2040 by weight, temperature
under heater during evaporation step was 520C, and gelation was in iced
water at about 20C.
Example 3 Membrane prepared as in Example 1 except that evaporation was
for 3 min. under heater at 650C.
Example 4 Membrane prepared as in Example 3 except that evaporation was
for 1 min.
Example 5 Membrane prepared as in Example 1 except that gelation was in
iced water at about 20C.
Example 6 Membrane prepared as in Example 1 except that the membrane
was not restrained on the plate by tape or any other means and
evaporation was at room temperature for 3 min.
To demonstrate that the membranes prepared according to the process of,
this invention have superior properties of flux and. retention of lower
molecular weight macromolecules, the following membranes were prepared
using solvents disclosed in the prior art. The resulting membranes were
evaluated as in Example 1. Results are shown in Table 2.
Example 7 A membrane was prepared following the same general procedure
as in Example 1 except that the casting dope was a 15% solution of Udel
P-1700 polysulfone in dimethyl formamide.
The evaporation of the cast film was carried out for 1 min.
under a heater at 680C. Gelation was in water at ambient temperature.
Example 8 A membrane was prepared following the same general procedure
as in Example 1 except that the casting dope was a 15% solution of Udel
P-1700 polysulfone in dimethyl acetamide.
The evaporation of the cast film was carried out for 1 min under a
heater at 38at. Gelation was in water at ambient temperature.
Example 9 A membrane was prepared as in Example 8 except that the
polysulfone concentration in the casting dope was 20% by weight.
Example 10 A membrane was prepared as in Example 8 except that the
temperature during evaporation was 65 C; Example 11 A membrane was
prepared as in Example 10 except that the casting dope contained 5t by
weight glycerol as a non-solvent or flux promoter in addition to the
15% polysulfone resin.
Example 12 A membrane was prepared as in Example 11 except that
evaporation was carried out at ambient temperature for 3 min.
TABLE I Example Feed* Flux Retention ml/cm² min.
1 1120 1.3 BD 0.3 100 C-20 0.1 99 C-6 0.1 81 C-l 0.7 1 2 H2O 0.24 BD
0.1 100 C-20 0.05 99.9 C-6 0.06 92 C-l 0.1 6 3 H2O 2.1 BD 0.2 100 C-20
0.07 95 4 H2O 2.4 BD 0.2 100 C-20 0.07 99 5 H20 1.6 BD 0.2 100 C-20
0.07 99 6 H20 1.7 BD 0.3 > 98 C-20 0.04 98 * BD = 0.18 aqueous Blue
Dextran 2000 solution (M. W. 2,000,000) C-20 = 1% aqueous Carbowax 20M
solution (M. W. 15,000-20,000) C-6 = 1% aqueous Carbowax 6000 solution
(M. W. 6,000) C-1 = 1% aqueous Carbowax 1000 solution (M.
W. 1,000) TABLE 2 Example Feed Flux Retention ml/cm² min. % 7 H2O 0.04
BD 0.04 > 98 C-20 0.02 94 8 H20 0.05 BD 0.03 98 C-20 0.02 98 9 H20 <
O.001 - 10 H20 0.001 11 H20 0.6 BD 0.09 96 C-20 0.03 88 12 H20 0.16 BD
0.11 97 C-20 0.05 94 In evaluating the properties of membranes it is
important that the membranes be thoroughly leached or rinsed with water
to remove-all residual solvents, preservatives, wetting agents, or
other contaminants before determination of flux and retention.
The membranes prepared according to the examples in this invention were
kept wet with water at all times prior to testing.
For determination of flux and retention the filtration is carried out
in a cell that provides flow of the test fluid across the surface of
the membrane to minimize concentration polarization, that is, build up
of a layer of concentrated solution at the surface of the membrane. An
Amicon TCF 10 ultrafiltration cell is an example of such a test cell.
For each determination of retention, a solution of a single compound
with a relatively narrow molecular weight range in water must be used.
If a mixture of materials, one having a molecular weight high enough
that it would ordinarily be retained by the membrane and the other
having a molecular weight low enough that it would not ordinarily be
retained to any significant extent by the membrane, were used, a higher
than normal retention of the lower molecular weight material would
likely be found because the molecules of the higher molecular weight
material that could not pass through the pores of the membrane would
build up at the surface of the membrane and impede the transport of the
lower molecular weight material to the membrane surface.
In evaluating the membranes prepared as described in the examples, the
water flux was determined at 30 psig with distilled water on a well
flushed membrane before the membrane was exposed to any test solution.
If the water flux is determined after exposure of the membrane to a
test solution, it may be lower than the initial water flux. Following
determination of the water flux, the flux and retention of the other
solutions at 30 psig was determined in the order listed. The membrane
was thoroughly rinsed with distilled water between each determination.
For determination of retention, four samples of permeate, each
containing at least 10 ml, were collected and analyzed separately.
Colorimetric analysis was used for Blue Dextran solutions and
gravimetric analysis for the Carbowax solutions. Retention was
calculated as R = ( conc. feed - conc. permeate ) 100 conc. feed There
was no significant difference or trend in the four results for a given
test solution and a given membrane.
Process Requirements The concentration of the film forming
non-sulfonated sulfone polymer dissolved in hexamethyl phosphoramide
solvent to form the solution for casting the membrane should be in the
range of 10 to 25 weight percent, preferably about 14 to about 18
weight percent The temperature of the solution should be high enough to
dissolve the polymer to form a clear solution but below the boiling
point of the solvent. Preferably, the temperature should be in the
range of about 85 to about 1000C.
The properties of the membrane are not significantly affected by their
formed thickness probably due to the fact that the membranes are
asymmetric and ultrafiltration takes place at a very thin "skin" or
layer of critical porosity at the top surface, the rest of the membrane
being more porous and offering little resistance to flow. The preferred
thickness of the film cast to form the membrane is from about 5 to
about 15 mils.
The solution of the sulfone polymer in hexamethyl phosphoramide is cast
on a smooth solid surface or substrate. The solvent is then allowed to
evaporate from the formed sheet but only to an extent necessary to
provide a material which will have the desired porosity after treatment
with the gelation medium. Evaporation may be brought about by exposure
of the formed (cast) sheet to air at room temperature for from about 1
to about 10 min.
or it may be hastened by increasing the temperature near the surface of
the film, for example by placing the film under an electric heater.
The gelation treatment, after the evaporation step, is required to
convert the liquid film into a solid membrane. The gelation medium is
kept at a temperature ranging from about -10 to about +500C, preferably
0 to about 250C, and the dwell time for exposure of the sheet to the
gelation medium must be suf ficient to solidify the membrane and
preferably at least about 1 min. The membrane may be left in the
gelation medium for a longer time if desired.
The cast film may be secured to the casting surface, for example by
placing tape along the outer edge of the casting plate, during the
evaporation and gelation steps, but this procedure is not necessary as
demonstrated by Example 6.