Note: Descriptions are shown in the official language in which they were submitted.
`` 1047719
This invention relates to microporous membrane filters composed
of polyesters derived from aromatic dibasic acids, especially polytetra-
methylene terephthalate and to a new process for forming microporous poly-
meric membrane filters. The new process, referred to as a polymer assisted
phase inversion, obviates the necessity for a surfactant in the membrane and
represents a new approach to the formation of microporous membrane filters.
The technology of synthetic polymeric membranes i9 quite well
developed in many areas and the literature on the subject is profuse. Probably
the best and most comprehensive treatment of this subject is the recent work
10 by the pre~ent inventor, Dr. Robert Kesting, entitled SYNTHETIC POLY-
MERIC MEMBRANES, McGraw-Hill Book Company, 1971, Library of
Congress catalog card No. 72-132345. The reader is referred in particular
~ to Chapter 5 of this work entitled "Porous Phase-Inversion Membranes" for
; a discussion of the principles of membrane formation.
Polyester resins and the terephthalate polymers are, in general,
well known. Polyesters were described by Carothers and Arvin, Journal of
the American Chemical Society, Volume 51, pages 2560-70 in 1929. See also
collected papers of W . H. Carothers on High Polymeric Substances, Volume I
HIGH POLYMERS, INTERSCIENCE PUBLISHERS INC., New York 1940.
20 Polyethylene terephthalate films have been marketed for several years under
~- the trademark MYLAR by DuPont and other polyester materials in various
forms have been available for several years.
Until now, however, microporous membrane filters of the polyesters
have not been available. One of the principle features of this invention
. . .
` involves microporous membrane filters composed of polyesters derived from
; aromatic dibasic acids. One of the more specific features of the invention is
the discovery and preparation of microporous membrane filters composed of
polytetramethylene terephthalate.
- - Another principle feature of the present invention is the provision of
30 a dry process for forming microporous membrane filters of aromatic dibasic
acid polyesters in which membrane formation is accomplished without the
necessity of the usual low molecular weight swelling agents and wetting agents,
.
.
- ' . ' :
' ` 104~719
the mem~rane for~ation being assisted by the inclusion of a
separate`and distinct polymeric compound. This dry process
is called a polymer assisted phase inversion process for
forming microporous membrane filters of aromatic dibasic
acid polyesters and is particularly applicable to polytetra-
methylene terephthalate and equivalent polyester resins.lie
- other significant features which will be discussed in detail
in the specification hereinafter.
Polyesters Derived from Aromatic Dibasic Acids
Polyesters which are derived from aromatic dibasic
acids, especially those derived from terephthalic acid, have
excellent chemical resistance, including resistance to acid
and base hydrolysis and resistance to a geeat number of organic
solvents.
In addition, these resins exhibit other outstanding
properties, such as high tensile strength, flexibility, and
thermal stability. For these reasons, this class of resins
represents an excellent a priori candidate for the formation
of microporous membrane filters. Until the present invention,
however, membranes of these materials which exhibit the structure
of the "typical" microporous membrane filters, such as those
prepared from nitrate and acetate esters of cellulose, and
certain polycarbonates (see Canadian copending patent 991,776
-~ etc., have eluded workers in the field.
; Polyesters prepared from the condensation of dibasic
acids and diols are well known as resins and possess the
~; following general formula:
.,
RC-8-R'-o-C 3n
wherein R is aryl and R' is typically alkyl, wherein n is integer
which may be as low as a few hundred to as high as several
~ thousand. - -
". :'
~ ~ - 2 -
'- :~-.:
. .
1047719
The only polyesters of interest in the present
invention are those in which the dibasic acid moiety is
aromatic, e.g. the benzene nucleus. The specific dibasic acid
of the greatest interest in the present invention is
terephthalic acid.
Thus, in accordance with the present teachings,
a process is provided for producing a microporous membrane~
The process comprises dissolving polytetramethylene terephthalate
polyester and polyvinyl alcohol in approximately equal amounts
in hexafluoroisopropanol. The solution is cast into a thin
film configuration and the hexafluoroisopropanol is subsequently
evaporated from the polymer. In addition, the polyvinyl
alcohol may be leached from inchoate pores in the polyester
to form a finished polyester microporous membrane.
-2a-
-,
` ` 10477~9
Ortho- and Isophthalic acid derived polymers are regarded as
equivalent only in the very broadest sense and only marginally suitable as
membrane formers in the present invention.
Polyesters of the class under consideration derived from tere-
phthalic acid haveJ within the context of the present invention, such unique
properties as to set terephthalate resins apart from other resins of this
clas s .
Diols which are suitable for the present invention are the allcyl diols
such as trimethylene glycol, tetramethylene glycol, pentamethylene glycol,
etc. Ethylene glycol derived polyesters of the class under consideration are
regarded a~ only very marginally suitable, and equivalent to tetramethylene -~
glycol derived polymers only in a broad sense, which would include usable,
but much less satisfactory membrane forming systems.
Generally, then, the most suitable polyesters are those having the
formula: `
O O
(2) ~ ~ -C-o-(cH2)a-o-c ~ n ~ :
wherein a is an integer from 3 to 5, most preferably 4, and marginally as
low as 2 or as high as 6 or 7, and wherein n is an integer greater than about
45 (MW-- 10,000) and preferably greater than 90 (M W-- 20,000). The most
., . :
suitable available range of polymers of this class thus far proved have an
n value within n = 90 (M W ~-- 20,000) to n = 225(M W-- 50,000), but the use
`i of such polymers having an n value of up to 450 (M W-- 100,000) or higher
will predictably permit preparation of High Void Volume membranes by the
~ dry process here disclosed, as well as by the process described in my co-
pending patent application referred to previously.
The range of effective equivalents, for purposes of this invention, ,
must, to some degree, be determined empirically, since a full range of
potential equivalents is not now available for evaluation and may not become
available goon enough to avoid undue delay in making this technology available
to the industry.
. ;:. .
-'
:. ..
-`` 10477~9
Some guidlines to potential equivalents are possible, however.
These are outlined briefly below.
Polyesters derived from ortho- or isophthalic acid are regarded as
potential, but significantly inferior, marginal equivalents suitable for some
applications if the solvating power of the casting solution solvent is reduced
and carefully controlled. (For use in the dry process herein disclosed, the
solvent must also dissolve the assisting polymer, thus making solvent
selection critical and unpredictable).
Polyesters of the class under consideration having the general
1 0 formula:
( ) ~-C-O-(CH2) a ~~C~n
wherein a and n are the integers as previously defined and wherein s is a
substituent which does not prevent polymerization of the polyester, e. g. a -
methyl or other lower alkyl group (1-4 carbon atoms), are also potential
equivalents and should permit the more effective use of diols having 2 carbon
atoms. Selection of a solvent having a lower solvating power, as discussed
previously is indicated for casting solutions of such polyesters, especially
where larger, or plural, alkyl substituents occupy one or more positions on ~ -
the aromatic dibasic acid nucleus.
Polyesters derived from fused polycyclic dibasic acids, for example ;
l :
(4) HOC ~COH
are not good candidates as equivalents, although the dry process described
herein could be used if a suitable solvent could be found.
Polyesters of the class under consideration derived from diols con- -
taining etheric linkage, for example
(5) HO-CH2CH2-0-CH2CH2-OH
and
(6) Ho-cHzcH2-o~o-cH2cH2-oH~
.~ ~047719
and from diols of high molecular weight, for example
(7) HO-(CH2)a'-OH
-
wherein a' is an integer of 7 or 8 or higher, are also not good candidates as
equivalents to the extent that membranes formed from such materials would
be less stable, less resistant and generally inferior and would require a
;~ different solvent system. The dry process disclosed herein could be used,
however, if a suitable solvent could be found.
Obviously, not all criteria for equivalence are set forth, and not all
parameters are known. I do not, however, wish to be limited to those
materials specifically set forth in the examples, nor do I wish to ignore the
fact that one particular combination of materials (polytetramethylene tere~
phthalate polyester, polyvinyl alcohol assisting polymer, and hexafluoro-
isopropanol) possesses such unexpected advantages as to be a distinct
discovery within the broader framework of this invention. ~
Until quite recently, the only commercially available dibasic acid ~ -
derived polyesters of the type under consideration here were the polyethylene
terephthalate polyesters prepared by the condensation of dimethyl tere~
phthalate and ethylene glycol. (Reference to dibasic acids and diols includes
polymerizable derivatives thereof.
Polymers of this type are sold in various forms under several ~ - -
trademarks including Mylar and Dacron.
I had postulated that if the alkyl chain were longer than that pro-
; vided by polyethylene terephthalate, polyesters of the type under consideration
would be excellent candidates for microporous membrane filter fabrication.
I have now proved my postulate correct and discovered that polytetramethylene
terephthalate prepared from the condensation of dimethyl terephthalate and
1 ,4,tetramethylene glycol does indeed form microporous membrane filters
having excellent chemical and physical characteristics.
A sufficiently high molecular weight polytetramethylene terephthalate
i~ available from Eastman Chemical Company under the trademark Tenite
Polyterephthlate, from General Electric Company under the trademark
-- 5 --
.: :
.. ....
- . ~ . , : . ~ -
-` - iO47~19 -.
--Yalox, and from the Celanese Corporation under the trademark
Celanex.
My familiarity with the process variables, the characteristics of
the products obtained and the chemical and physical properties of the
materials involved indicate with relative certainty that polyesters derived
from aromatic dibasic acids (or polymerizable derivatives, e. g. esters,
thereof) having 3 to 6 or 7 methylene groups in the alkyl moiety of the
polymer can be fabricated into good to excellent quality microporous mem-
brane filters using ~he techn;ques and processes of this invention as described
hereinafter .
In general, any polyester derived from an aromatic dibasic acid
which is insoluble in water, possesses acceptable physical properties (is
self-supporting at ambient temperature and is 6ufficiently dimensionally
stable), and is soluble within the range of 2% to 20% (by weight) in combina-
tion with an assisting soluble polymer which also has solubility in the 2% to
20% (by weight) range in a solvent which has a boiling point in the ambient
to approximately 150C range must be regarded as an equivalent in the
broadest sense, -but not in the narrow sense of being interchangeable with
polytetramethylene terephthalate with like results in the same casting solution
system. Indeed, I regard only polytrimethylene terephthalate and poly-
20 pentamethylene terephthalate polyesters as substantially certain to be closely
equivalent and functional in this latter sense.
The preferred polyester, polytetramethylene terephthalate, sold
under the trademark Ten*e Polyterephthalate 6 PRO, ha~ outstanding
chemical properties illustrated by Table I which shows the effects of various
chemicals on this grade of polytetramethylene terephthalate.
% Increase After One Year
Chemical In Weightln Thickness
Acetic Acid (Glacial) 2. 65 1. 51
Acetic Acid (5%) * 0. 40 - 0. 29
Acetone 5. 56 2. 57
Ammonium Hydroxide (Conc . ) O. 80 -O. 16
Ammonium Hydroxide (10%)0.49 _0.53
"
)47719
Aniline 6.69 4.17
Benzene 3.26 2.19
Carbon Tetrachloride0.72 0.42
Chr omic Anhydr ide (40 %) 0.10 - 0.11
Citric Acid (10%) 0.37 0.16
- Cottonseed Oil 0.09 -0.16
Alconox Detergent (0.25%) 0.37 -0.34
Diethyl Ether 0.95 0.45
Dimethyl Formamide 2.48 1.56
10 Deionized Water 0 39 0 53
Ethyl Acetate 3.30 1.93
Ethylene Dichloride 18.66 6.16
Heptane 0.13 -0.26
Hydrochloric Acid (Conc. ) 0.56 0.03
Hydrochloric Acid (10%) 0.33 _0.05
Hydrogen Peroxide (28%) 0.41 0.00
Hydrogen Peroxide (3%) 0.38 -0.05
Isooctane 0.08 -0.08
Kerosene 0.10 -0.16
20 Methanol 1.61 0.16
% Increase After One Year
.
Chemical In Weight In Thickness
Mineral Oil 0.06 - 0.13
Nitric Acid (Conc. ) -disintegrated-
Nitric Acid (40%) 0.68 0.05
Nitric Acid (10%) 0.36 -0.16
Oleic Acid (93%) 0.26 -0.18
Olive Oil 0.06 0.11
Phenol (5 % in water ) 9.48 4.88
30 Sodium Chloride (10%)0.32 0.05
Sodium Carbonate (20%) 0.31 0.05
Sodium Carbonate (2%) 0.36 0.00
~' ~
`~ 477~9
.,
Sodium Hydroxide (10 %)-1.20 -1.16
Sodium Hydroxide ( 1 %)0.37 0.77
,r Sulfuric Acid (Conc. ) -disintegrated-
Sulfuric Acid (30%) 0.25 - 0.52
- Sulfuric Acid (3%) 0.36 0.00
Turpentine 0.27 -0.03
Toluene 2.35 1.40
Sodium Hypochlorite (3.5%) 0.37 -0.11
Ethanol 0.48 0.21
10 Ethanol (50%) 0. 47 0.03 ~ -
Dibutyl Sebacate 0.07 -0.05
Di-2-Ethylhexyl Phthalate0.07 0.08
Ivory Soap (1%) 0.37 0.08
Brake Fluid -
Transformer Oil 0.06 -0.18
* All percentages are concentrations in water.
These membranes are extremely tough, flexible, dimensionally stable over
a broad range of temperatures, maintain their strength at both high and low
temperatures and possess other highly desirable physical characteristics.
The Polyester Membrane Product
In terms of a manufacture or product, the present invention is
directed to a polyester derived from a dibasic acid, as discussed herein- ~ - -
before, which is in the form of a microporous membrane. -
The microporous membrane of this invention is composed of a
polyester derived from a dibasic acid and may include a hydrophilic second
polymer or a second polymer which is soluble in a fluid which is a non-
solvent for the polyester.
Typically, the membrane of this invention, in finished form, is
composed of a dibasic acid derived polyester, as discussed previously, and
30 a minor proportion of the second polymer, which is hydrophilic or soluble
in a polyester non-solvent fluid, or includes, as a component, such a
membrane composition.
104`7719
The membranes here disclosed may be used as membrane filters
in microfiltration, ultrafiltration, in reverse osmosis, as reverse osmosis
membranes or as porous support substrates for other reverse osmosis
membranes, the cellulose acetate membranes, for example, and in electro-
phoresis, curtain electrophoresis, thin layer, gas and liquid chromatography
and as substrates to which enzymes and other catalysts can be attached for
solid-liquid reactions. The porous membrane filters in this invention may
be produced in flat sheets, tubular forms on the inside or the outside of the ~--
tube, as hollow fibers, and in microporous bead configurations. These
10 microporous membrane filters are of special significance to the production - -
of microporous tapes because of their strength and flexibility.
One particularly unique advantage of the subject membrane system
results from the fact that the assisting polymer fills the incipient voids in the - -
unleached membrane. The surface of the unleached membrane is, therefore,
still amenable to fabrication of multilayered membrane structures, with one
membrane layer being used as the substrate for the next layer. This permits,
inter alia, production of very thick membrane sections. -
Other advantages possessed by the membranes of this invention are
the absence of surfactants and of a tendency to build up static charges which,
20 in conventional microporous membranes, tracks dust and makes handling
difficult and results sometimes uncertain.
Other and more specific characteristics of the product will be
apparent from the discussion of the process which follows and from the
subsequent references to the product . ~
The Polymer Assisted Phase Inversion Process - - - -
.
These membranes are fabricated by forming a casting solution of
a dibasic acid derived polyester, a second polymer which is soluble in a fluid
- which is a non-solvent for the polyester, and a liquid which is a solvent for
both the polyester and the second polymer. This forms the casting solution
30 for forming the membranes. The casting solution is then cast upon a sub-
strate to form a film of desired dimensions. The solvent is permitted to
evaporate from the film and at least a portion of the second polymer is
_ g _
.~ , .
`;``` 1047719
leached from the film with the fluid which is a non-solvent for a polyester.
This leaves a microporous membrane which has excellent chemical and
physical characteristics, the characteristics of which can be varied by
controlling the nature and amount of the second polymer in the membrane,
as well as by the selection of the polyester, and by control of process
variables .
Polytetramethylene terephthalate is considered to be the first truly
suitable aromatic dibasic acid based polyester for formation of microporous
membrane filters because of its greater solubility (compared to polyethylene
terephthalate) in suitably volatile solvents. This property makes it possible
to prepare true, molecularly dispersed, solutions of polytetramethylene
terephthalate in a few selected volatile solvents. Polytetramethylene tere-
phthalate is more soluble (i.e. exhibits a higher degree of dispersion), than
polyethylene terephthalate, probably because of the greater flexibility of the
polymer chains, owing to the presence of four methylene groups in the former,
rather than the two separating the terephthalate acid moieties in the latter. -~
Further, and of distinguishing importance, polytetramethylene
terephthalate rèpresents a very close approach to, if not the actual accom- ~`
plishment of, the ideal balance between sufficient solubility to permit fabrica- ~;
tion into membranes, and sufficient solvent resistance, to permit~utilization
of the resultant membrane in the filtration of solutions in common organic
solvents. By comparison, polyethylene terephthalate is insufficiently soluble
to permit fully satisfactory membrane fabrication. On the other hand, the
inclusion of too many or too large substituents on the aromatic nucleus of the `
diacid or too long methylene chains or etheric links in the diol, or other
substitution or modification which greatly increases the solubility of the poly-
ester degrades the chemical and physical resistance of the resultant polyester
and may require significant modification of the membrane fabrication process.
The first partially successful prototype membranes prepared
according to this invention were from polyethylene terephthalate solutions.
Such solutions are turbid, whereas polytetramethylene terephthalate solutions
are clear indicating a finer dispersion in the latter solutions. Fine dispersion
- 10 _
``` ~ 1047719
in the solution, and hence clear solutions, are highIy desirable
and are probably mandatory in any commer~ially feasible process,
- since only the latter clear casting solutions are sufficiently
stable for utilization in the dry phase inversion process on a
commercial scale~
The reader is referred to Keating, SYNTHETIC POLYMERIC
- MEMBRANES, referenced previously and my Canadian Patent 991,776
entitled INTEGRAL UNSKINNED HIGH VOID VOLUME POLYCARBONATE
MEMBRANES AND A DRY PROCESS FOR FORMING THE SAME, for a detailed
discussion of the phase inversion process for fabricating
polymeric membranes. The broad principles of phase inversion
membrane formation are applicable to the present dry process;
however, the present process is believed to be the first --
satisfactory example of a phase inversion process for manufacturing -
microporous membranes in which the swelling agent consists
of a solvated second polymer which assists in the phase inversion
in a manner similar to the manner in which typical swelling agents
of low molecular weight assist in the phase inversion in common
phase inversion membrane formation. For this reason, the process
~ is referred to as a Polymer Assisted Phase Inversion Process,
sometimes abbreviated "PAPI" process.
; The process consists in dissolving the polyester
and one or more, usually only one, second polymer(s) in a volatile -
solvent, (i.e., a solvent which has a boiling point below about
150C and preferably below about 100C~ or in a combination of
solvents, swelling agents and non-solvents for both the polyester
and second p~lyester cast ng~the solution as a film, and
evaporating the solvents from the solution. The process can be
either wet or dry; that is, it may involve either partial or
complete evaporation, the dry process being preferred whenever
applicable.
)r -
~ .
~ - ,
)47719
The ove~all process involved herein is described as
a poiymer assisted phase inversion process but it has elements
of a leachin~ process as weIl. rn those cases where a non-
polyester polymer component is either desired or does not
interfere with the end use of the filter, these non-polyester
polymer components may simply be left in the microporous
membrane. In cases where the second polymer, i.e., the non-
polyester polymer, would be objectionable.
,~
~ - lla -
047719
the second polymer may be leached out with a solvent for the second polymer
which is a non-solvent for the polyester.
The process may be varied to form polymeric microporous mem-
brane filters for specific end uses. For use in microfiltration, ultrafiltra-
tion and electrophoresis, curtain electrophoresis, gas, liquid and thin
layer chromatography, an unskinned (i.e., a more or less symmetrical
membrane) is desired. In these cases the dried membrane may be either
leached of its non-polyester component or components or may be used with ~ ~ -
the second polymer in place, depending upon the specific requirements of
the end use. It is also possible where two or more second non-polyester - `
polymers have been included, to leach one or more of the polymers from the -~
membrane, leaving behind one or more polymers together with a polyester.
Likewise, it is possible to employ in situ cross linking to permanent-
ly fix one or more otherwise leachable second polymeric components in the --
membrane.
Included second polymers may function to improve wettability of
the polyester in one of several ways. They may be surfactants or they may
simply possess an affinity for water, up to and including water solubility.
Where the included polymers are not actually surfactants an important new -
dimension has been added to membrane filters, since the presence of sur- ~ -
factants, present in commercially available membrane filters, is often
deleterious to certain biological separations, and in handling foodstuffs, waterfor drinking purposes, etc. Therefore, one important feature of the invention
i8 that wettability can be obtained by the inclusion of either leachable or,
when necessary non-leachable hygroscopic, but not surface active, polymers.
Two types of asymmetric, i.e., skinned, reverse osmosis mem- -
branes and two separate processes for their preparation have been described.
These are the Loeb-Sourirajan wet process and the Kesting dry process.
Both of the~e processes use cellulose acetate polymers in their manufacture.
Cellulose acetate is, however, rather friable microporous membrane - - -
configurations. Stronger and less friable reverse osmosis membranes are,
therefore, desirable.
- 12 - -
: -
:``` 10477~9
This difficult goal is accomplished, in the present invention, bycasting a membrane either a thin dense or a thicker asymmetric membrane,
of cellulose acetate or other polymer (such as the polyester itself), suitable
for reverse osmosis upon the polyester membranes prior to leaching, so
that the polyester membrane after leaching serves as a porous backing
support for the cellulose acetate or other reverse osmosis polymeric
membrane. Adhesion of the reverse osmosis membrane to the microporous
support can be achieved by including, as a constituent of the casting solution
~ .. ., . _
for the reverse osmosis membrane, a solvent for the polyester. When
10 properly balanced (containing the correct amount of polyester solvent~, this
solvent system will sufficiently soften the surface of the polyester to permit
good adhesion without destroying the porous characteristics of the polyester
membrane support. Following the application of the second layer, the
leachable assisting polymer of the substrate may be removed by washing
with water, alcohol or other polyester non-solvent.
It is noteworthy also that dense polyester membranes themselves
have the ability to function as reverse osmosis membranes, as do asymmetric
polyester membranes having a dense skin on a porous substructure.
Solvents
Solvent selection is extremely critical. Indeed, to date only two
solvents, hexafluoroisopropanoI and trifluoroacetic acid, are known to be
acceptable for the dry process. The wet process is less desirable but the
same solvents can be used in such a process.
There are no known equivalent solvents for use with polytetra-
methylene terephthalate polyester of suitable molecular weight for membrane
fabrication, and one has little basis for making reliable predictions as to
potentially equivalent solvents. Probably only highly fluorinated substituted
low molecular weight (1-4 carbon atom) alcohols, acids, ethers, esters and
acids can now be considered as solvent candidates in the preparation of the
30 subject membranes, and it is known that not all such compounds are satis-
factory. Trifluoroethanol and the hexafluoroacetone-methanol hemiacetal,
for example, are not satisfactory solvents for fabricating polytetramethylene
~ 1047719
terephthalate membranes. rndeed, even trifluoroacetic acid is
so vastly inferior to hexafluoroisopropanol as to make the latter
a compnent of a distinct discovery within the broad inventive
concept here disclosed.
It may be suggested that where less solvating power
is required, as with substituted aromatic dibasic acid or longer
chain diol derived polyesters, other highly fluorinated compounds
of the class discussed would be suitable, but ultimate solvent
selection must, at present, be on an empirical basis.
; 10 Any d~sired solvating power, within the range of the
solvents, may be obtained by mixing selected solvents. Hexaflu-
oroisopropanol and trifluoroacetic acid, for example, may be mixed ~ --
or diluted with other so~vents of the class under consideration.
It must be understood that solvent selection is not -
alone a function of the nature of the polyester. The nature of ~ -
the assisting polymer and, more correctly, the nature of the
combination of polyester and assisting polymer are also controlling,
or at least very influential in solvent selection.
- Assisting Polymers
Suitable sécond or "assis~ing'l polymers are very rare.
They include certain hygroscopic polymers, polyvinyl alcohol in
partic~lar. Polyvinyl pyrrolidone also functions, but is much
less suitable than polyvinyl alcohol. These polymers can be
softened and/or leached in water. Other polyers which are
mutually solvent together with the polyester in a solvent, and -
which are soluble in a polyester non-solvent are candidates --
as assisting polymers.
In a more general sense the assisting polymers must be
soluble in a solvent which is a non-solvent for the polyesters
and must also be soluble in a mutual solvent for themselves and
the polyesters.
- 14 -
-` 1047'719
.:
~ utual solubility ~f the type discussed here, in
approximateLy equal weights of the polyester and the assisting
polymer at the high totalsolids content, i.e., between approximately
5 and 20%, necessary for the practical production of membranes
is extremely rare in most solvent systems and is made possible
by the ususual solvent properties of the p~lar fluorinated
compounds wh~ich are employed as mutual solvents. Because of strong
, 10
- 14a -
;
-- 1047719
solvation, solvated chains of dissimilar polymers are apparently much less
incompatible than they would be in the absence of solvation.
The void volume of the membrane produced by the polymer assisted
phase inversion process can be controlled by varying the ratio of the second
(leachable) polymer to the polyester. The higher the ratio, the higher the
void volume. For the polytetramethylene terephthalate polymers of the
molecular weights available today, e. g., from about 25,000 to 50,000, void - -
volumes above about 60% are not achievable with the maintenance of the
excellent physical properties found to date. It is to be predicted, however,
as higher molecular weight polyesters become available higher void volumes
will be achievable using the polymer assisted phase inversion process.
The pore size and void volume of the membranes can be controlled
by controlling the nature of the second, "assisting", polymer. The most
suitable assisting polymer found to date, is polyvinyl alcohol. -
Pore size is a function of the molecular weight and acetyl content
of the assisting polymer. High (37-42% by weight) acetyl content (e. g.
Gelvatol*40-10) polyvinyl alcohols yields coarse structures. Low (0-4%
by weight) acetyl content resins (e. g. Gelvatol * 1-30 and 1 -60) yield fine
structures. Intermediate size structures (void and pore size ~ 0.05,um to ~ -
1,0.2,1lm) are obtained with intermediate (19.5-22.7% by weight) acetyl content
(e.g. Gelvatol* 20-30 and 20-60). Fineness also increases with increasing
molecular weight although less dramatically than with decreasing acetyl
content. In general the best physical properties are obtained by employing
resins of intermediate (20 + 5% by weight) acetyl content and intermediate
(10,000 to 14,000) molecular weight (e.g. Gelvatol* 20-30) resins.
Polyvinyl pyrrolidone also functions as an assisting polymer, but
gives such vastly inferior process and product result, as compared with
polyvinyl alcohol, as to make the use of the latter material a distinct and
very superior discovery within the broad concept of the invention.
* Trademark.
- 15
.
. ` ` .
`" 1~47719
Example 6
Example I
A solution consisting of 5~oby weight, Eastman Tenite* Polytere-
phthalate 6 PRO polytetramethylene terephthalate, 5% Monsanto Gelvatol*
20-30 DP polyvinyl alcohol, and 90% hexafluoroisopropanol was cast on a
glass plate to a thickness of 0. 020 inches and dried to completion. The
result was a 0. 003 inch thick microporous membrane which was rapidly wet
by water which rapidly leached the polyvinyl alcohol from the membrane.
The membrane was homogeneous, opaque, strong and flexible. It
10 exhibited essentially the same solvent resistance as the bulk form of the
material and is, therefore, applicable to filtrations in aqueous and in non-
aqueous media. The pore sized distribution was very narrow, with a median
value of about 0.1 ,~m (a bubble point of about 90 psi). The final membrane
was a highly satisfactory unskinned microporous membrane filter having
physical and chemical characteristics vastly superior to cellulose acetate
and other membranes which have been previously reported. These homogen-
eous unskinned membranes are useful in microfiltration, ultrafiltration and
electrophoresis, curtain electrophoresis, and in gas, liquid and thin layer
chromatography. For example, in electrophoresis a membrane produced th~
20 standard 5-line (albumin, (1~ C~-2- B andy ) spectrum for serum proteins.
The application artifact was small and outside of the spectrum when used with
the Beckman* Microzone cell (see, Microzone Electrophoresis System
Brochures of the Beckman Instrument Company, Fullerton, California).
The bubble point of a membrane mentioned above is a means of
determining pore size. See ASTM D-2499 which discusses the essential re-
lationship between pressure, surface tension of a wetting fluid, and pore
diameter. This relationship predicts that a wetted membrane with applied
differential air pressure will permit no flow of pressures below a critical
level referred to as the "bubble point". At the bubble point, wetting fluid is
30 forced out of the smallest pores and flow thus begins. With pore size
measured in micrometers (10-6 meters), pressure P, measured in pounds/
square inch, and with kerosene as the wetting fluid, the relation between the
pore size and pressure is ~A = 12 5
* Trademark
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Example II
A solution consisting of 5% Eastman Tenite * Polyterephthalate 6
PRO polytetramethylene terephthalate, 5% polyvinylpyrrolidone (molecular
weight approximately 10,000) and 90% hexafluoroisopropanol was cast to a
thickness of 0. 020 inch on a glass plate and evaporated to dryness. The
resultant membrane consisted essentially of a porous polyester substructure
whose pores were filled with polyvinylpyrrolidone. A 1% solution of
cellulose acetate in a solvent consisting of 49. 5% hexafluoroisopropanol and
49. 5% methylene chloride was cast on top of the unleached polyester mem- -
10 brane, forming a thin adherent cellulose acetate film. The composite
membrane consisting of a cellulose acetate skin and a porous polyester
support was strong and flexible and was found suitable for reverse osmosis
applications. The polyvinylpyrrolidone was leached from the polyester
membrane as in Example I.
Exam~le III -
A solution consisting of 5% Eastman Tenite * Polyterephthalate 6
PRO polytetramethylene terephthalate, 5% polyvinyl alcohol Gelvatol* 20-60
(molecular weight approximately 86,000), acetyl content approximately 20%, -
and 90% hexafluoroisopropanol was cast to a thickness of 0. 020 inch on a
20 glass plate, and forcibly evaporated with a stream of hot dry air from a hair
dryer. The resultant membrane was asymmetric as evidenced by the presence
of a glossy skin which would not wet when a drop of water was placed there-
upon and a matte finish, porous substructure which readily wet. This
membrane was suitable for reverse osmosis and yielded a product flux of
2 gfd (gallons/ft2 day? and 28% salt rejection from a 5,000 ppm NaCl feed
solution at 800 psi and 25C.
Example IV
A solution consisting of 5~0 Eastman Tenite * Polyterephthalate 6 PRO
polytetramethylene terephthalate, and 95% hexafluoroisopropanol was cast and
30 allowed to evaporate completely yielding a gray dense membrane . 001 inch
thick. This membrane was utilized in liquid permeation as follows: A charge
* Trademark
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containing 95% methane and 5% n-pentane at a temperature of -150C and a
differential pressure of psi was found to permeate the membrane at a rate
of 144 grams /ft2 hr yielding a per meate c ontaining 98 . 5 % methane and l . 5 %
n-pentane. Such membranes are especially useful for this application when
formed into the hollow fiber configuration.
Example V
A solution containing 0. 5 to l. 0 percent Eastman Tenite * Poly-
terephthalate 6 PRO polytetramethylene terephthalate and 0. 5 to 1. 0 percent
polyvinyl alcohol Gelvatol* 20-30 and 98 to 99% hexafluoroisopropanol was
lO sprayed from an aspirator nozzle to yield fine droplets which quickly dried
to a fine powder which when leached with water yielded microporous beads
suitable for use in column chromatography for separation of, e.g., protein
solutions. If denser powders or powders are desired, higher concentrations
(similar to that described in Example I) may be employed.
Example VI
The solution described in Example I was cast 0. 020 inch thick onto
a Mylar * backing yielding, after leaching, a microporous membrane
intimately bonded to the Mylar *. This supported membrane yielded the
typical 5 line spectrum for serum proteins when employed in the Beckman*
20 Microzone Electrophoresis System and was also suitable for thin layer
chr omatography .
Example VII
The solution described in Example I was cast 0. 020 inch thick on a
glass plate and allowed to dry completely at which time another membrane
was cast on top of it and allowed to dry completely. By repeating this
procedure three times a 0. 009 inch thick membrane was produced. This
process can be repeated as many times as necessary to produce a membrane
with any desired thickness. Moreover, a membrane of e.g. coarse porosity
can be coated onto one of fine porosity to produce a laminated prefilter-
30 filter composite membrane in one piece. Laminates of dense membraneswith porous membranes have been described in Example II.
* Trademark
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General Discussion_
Microporous membranes suitable for ultrafiltration, microfiltration
and electrophoresis, reverse osmosis, liquid permeation, curtain electro-
phoresis, gas, liquid and thin layer chromatography, are fabricated from
polyesters produced by the condensation or aromatic dibasic acid and alkyl
diols by a polymer assisted phase inversion process.
The limitations of the polyesters are that they must have at least as
great a flexibility and solubility as polyethylene terephthalate and preferably
the degree of flexibility and solubility possessed by polytetramethylene
terephthalate. The polyesters must be formed from the condensation of an
aromatic dibasic acid and a diol, polyesters formed by the condensation of
an aromatic diol and an alkyl dibasic acid being unsatisfactory. Molecular
weights in the range of 25, 000 to 35, 000 are quite suitable but the molecular
weight range is not so critical as in phase inversion processes employing low
molecular weight swelling agents. The polyester must be soluble to the extent
of at least about 0. 5% by weight and preferably greater than lyo or 2% by
weight ~n a fluorinated compound such as hexafluoroisopropanol or trifluoro-
acetic acid. -`
The second (phase inversion assisting) polymer must be mutually
soluble together with the polyester in one or more of the fluorinated solvents ~ -
and must also be soluble in the polyester non-solvent. The assisting polymer
must be mutually soluble in one of the fluorinated solvents to at least about
0. 5~0 by weight and preferably greater than 1% or 2% by weight in an amount
nearly equal to or greater than the weight of the polyester which is dissolved
in the casting solution. The solubility of the assisting polymer in the polyester
non-solvent may be less or more than its solubility in the polyester solvent,
depending upon the end use characteristics. For some end uses, only gross
swelling of the second polymer is sufficient "solubility" where the second
polymer is to remain in the membrane. In other instances, where the second
polymer is to be removed essentially completely from the membrane, the
second polymer must be highly soluble in the polyester non-solvent. There is
no lcnown reason other than the improbability of compatibility why 2, 3 or even
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more 'lsecondllpolymers may be included, along with the polyester, in the
casting solution, although such systems have not been explored. One or
more of these may be leached out by appropriate selection of non-solvents
for the polyester but which are solvents of selected strength for the second
polymer which is to be leached. The nature of the second polymer, e.g.
polyvinyl alcohol, can be varied, e.g. as to molecular weight and acetyl
content, to influence the pore and void size and physical properties.
The water soluble polymers, polyvinyl alcohol and polyvinyl
pyrrolidone especially the former, are of greatest interest simply because
10 of the great utility of aqueous leaching media industrially, clinically, and in
the laboratory, but the invention is not limited to the use of second polymers
which are soluble in water or other hydroxyl containing solvents, and suitable
second polymers may be selected on the basis of solubility as set forth
hereinbefore .
Only hexafluoroisopropanol and trifluoroacetic acid are known to be
entirely satisfactory casting solution solvents, i.e., have adequate solvent
strength for polyesters and for the second polymers, proper boiling point and
volatility, and result in strong coherent microporous membranes. However,
the various hydrates of hexafluoroacetone may also be acceptable under
20 certain extreme conditions.
The casting solution for forming the microporous membranes of
this invention should contain at least about 2 and preferably from 5 to 20% by
weight, of the resins, i.e., the total polyester and second polymer resin
content should be, preferably, greater than 5% by weight of the casting
solution. Typically, no surfactant is required in the casting solution, thus
obviating many of the difficult problems which have plagued manufacturers
and users of microporous membranes in the past. ~ -
Extreme care and highly controlled evaporation and casting condi-
tions are required to form even moderately satisfactory microporous
30 membranes using polyethylene terephthalate; however, casting solutions of
polytetramethylene terephthalate are highly stable and the constitution of
these casting solutions can vary over a wide range of concentration and the
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casting can be performed under a broad range of temperatures and atmos-
pheric conditions. It is usually desirable for health and economic reason~
to cast the microporous membranes in an enclosed hood system to prevent
exposure to the solvents and to permit recovery of the solvents for reuse.
Other than these fairly simple precautions, no critical casting or proces~
conditions have been observed.
Leaching of the second polymer can be done simply by immersing
the membrane in the leaching solvent, e.g., water or an aqueous solution.
Leaching will, of course, be accelerated when the leaching solution i~
agitated to provide constant washing of the membrane. Time, temperature - `
and process variables do not appear to be critical in either the casting or
the leaching process.
Casting as used here includes the formation of any large surface -
area configuration, such as sheets, ribbons, particles, etc.
Summary
:- A fundamental polymer assisted phase inversion process has been
- discovered and disclosed which includes the following steps: a solvent
resistant polymer, for example the polyesters discussed herein, and an
assisting polymer are dissolved in a mutual solvent to a weight percent
20 concentration of from about 2% to about ZO% to form a membrane casting
solution. The casting solution is dried under conditions such that a large
surface area is exposed, e. g. in the form of a thin film or small particles.
The mutual solvent can be extracted in other ways also, as in the well known
wet process. This results in a membrane (using the term broadly to include
small particles such as chromatographic micro-spheres) in which the inchoate
voids in the solvent resistant polymer phase are filled with the assisting
polymer. The membrane is then exposed to a solvent (in the broad sense
discussed previously) for the assisting polymer which is a non-solvent for
the solvent resistant polymer. This exposure may only solvate or swell the
30 assisting polymer, but in most practical applications the assisting polymer
will be leached partially or completely from the membrane to leave finished
porous membrane. This leaching may be performed in preparation for use
or as an incident to the desired end use.
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Another fundamental discovery here disclosed is a polyester mem-
brane composed of a polyester having the following general formula
C-O-~CH2)a -O-C~
wherein a is an integer from 2 to 7, preferably 3 to 5, and n is an integer
from about 45 to greater than 225 (corresponding to a molecular weight of
from about 10,000 to about 50,000) and preferably between about n=90 and `'
n=225 (MW ~- 20, 000 to MW - 50, 000) .
Lying within, and exemplifying, the fundamental process discovery
is a process for forming polyester membranes which could not have been
expected or predicted even if the more fundamental process had been known
previously. According to this process, a polyester having the formula
O O '.:
~-C-O-(CHz)a -O-C ~n
wherein a is from 3 to 5 and n is preferably from 90 to 2Z5 or greater and
an assisting polymer selected from the group consisting of polyvinyl alcohol
and polyvinyl pyrrolidone or mixtures of these are dissolved to a total weight
20 percent of from 2% to Z0% (preferably about 5~0) in a solvent which consists
essentially of (i.e. the solvating power for the polymers of interest is not
substantially reduced by diluents) hexafluoroisopropanol, trifluoroacetic acid
or mixtures of these solvents. The polyester and assisting polymers are
present in approximately equal volume percent (of the dry membrane) con-
centrations in the casting solution i. e. the polyester and assisting polymer
are present in about 50 v/o + 10 v/o concentrations in the membrane as cast,
before leaching. Generally, there will be a greater amount of assisting
polymer in the casting solution on a volume percent basis. The process is
carried out as in the fundamental process. It is recognized that the foregoing ! `
30 may be the only possible examples of the fundamental process, but this has
not been established.
Within the foregoing examplification of the fundamental process is ,1
a particular process which is so unexpectedly and strikingly superior that it
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constitutes a distinct major discovery which will have far reaching .qcientific
and industrial consequences. It has been discovered that one, and only one,
combination of constituents, when used in the polymer assisting phase
inversion process under consideration, gives greatly superior results with
greatly reduced process problems and significantly increased reliability.
These constituents are: ~
polyester: ~3 c o-(CH2)a ~~C~n
wherein a is 3 to 5 (preferably 4) and n is 90 to 225.
assisting polymer: polyvinyl alcohol having a molecular weight of from
about 5,000 to about 25,000 (10,000 to 14,000
preferred) and an acetyl content of from 0 to about
50% (15% to 25% preferred).
mutual solvent: hexafluoroisopropanol
leaching solvent: water (or any solvent for polyvinyl alcohol which is
a non-solvent for the polyester).
- Of these constituent materials, only the leaching solvent is not critical to the
achieving of the vastly superior results obtainable by this process.
The product of the foregoing superior process, i.e. the resulting
polyester membrane, constitutes a distinctive and unique discovery not
expected and incapable of prediction, even in vlew of the more fundamental
discoveries .
It is apparent, then, that this invention is both general and specific.
It is general in the sense of the more fundamental aspects of the process and
resulting polyester membrane. It is specific with respect to the unique,
unexpected and superior results (as compared with the more general results)
which accrue from using the particular combination of the indicated polyester,
polyvinyl alcohol and hexafluoroisopropanol in the process.
The following claims are drafted with these general, certain inter-
mediate features and the specific features of the invention in mind.
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