Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~lSlC~
This invent~on pertains to methods for preparing
anisotropic membranes, especially hollow fiber membranes.
Particularly attracti~e aspects of this inYention include
methods for preparing polysulfone hollow fiber
S membranes suitable for the separation of gases in which
the material of the hollow fiber membrane effec~s
separation by selective pe~meation and anisotropic
polysulfone hollow fiber membranes.
The viability of the use of membranes for fluid
separations as compared to other separation procedures
such as absorption, adsorption, and liquefaction often
depends on the cost of the apparatus and its operation
including energy consumption, degree of selectivity of
separation which is desired, the total pressure losses
caused by the apparatus for conducting the separation
procedure which can be tolerated, the useful life of such
apparatus, and the size and ease of use of such apparatus.
Thus, membranes are soug~t whic~ provide desired
selecti~ities of separation, fluxes and streng~h.
Moreover, in order to be commercially attractive on an
economic basis, the membranes are prefera~ly capable of
being manuactured in large quantities while achieving
a reliable product quality and bèing readily and
relatively inexpensively assembled in a permeator,
Particularly advantageous membranes are unitary
anisotropic hollow fiber ~embranes which have a relatively
thin layer Coften referred to as separating layer, barrier
layer, or active layer) integral with a porous structure
which provides support to the separating layer and of~ers
' ~P
- -` 1141510
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little, if any, resistance to the passage of fluids. In
order to prepare these integral anisatropic membranes, a
unitary membrane structure must be formed which possesses
diametrically opposed structures. The separating layer
must ~e formed such that it is thin and possesses few,
if any, pores or other defects. On the other hand, the
conditions which make the integral anisotropic membrane
must also provide a support structure which is highly open
suc~ that it offers little resistance to fluid flow.
Membranes have been prepared in film and in hollow
fiber form. Numerous proposals have been made pertaining
to the preparation of integral anisotropic mem~ranes in
film form. In general, anisotropic film membranes are
prepared by casting a solution of the polymer to form the
membrane in a solvent onto a surface, e,g. t a polished
glass surface. The polymer may be allowed to coagulate,
at least paxtially, in air or a gaseous or vaporous
environment and then it is usually immersed into a liquid
coagulant. Considerable flexibility exists in preparing
anisotropic film membranes. For instance, since the
polymer solution is placed on a support, the membrane
precur~or structure need not be self supporting at least
until after coagulation is completed. Similarly, since
one surface of the cast mem~rane is in contact with the
support, each side of the membrane may be subjected to
diferent coagulation conditions thereby permitting
substantially different structures to be achieved at each
surface of the membrane. Accordingly, membranes having a
relatively thin layer having an essential absence of pores
may be achieved at one surface of the film membrane, while
the remainder of the membrane may be relatively porous.
Moreover, since the film membrane precursor is supported,
the coagulation conditions including coagulation times,
can be widely varied to achieve the desired film membrane
structure.
In some instances, however, film membranes may not
be as attractive as other gas separation apparatus due
to the need for film mem~ranes to be supported to
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5~0
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withstand operating conditions and the overall complexity
of apparatus containing film membranes. Membranes in the
configuration of hollow fibers may overcome some of the
deficiencies of film membranes for many separation
operations. The hollow fibers are generally self-
supporting even under operating conditions, and can
provide a greater amount of membrane surface area per unit
~ol-ume of separation apparatus than that which may be
provided by film membranes. Thus, separation apparatus
containing hollow fibers may be attractive from the
standpoint of convenience, in size and reduced complexity
of design.
Many different considerations are involved in making
a film m~mbrane than are involved in making a hollow fiber
membrane. For instance, no solid support, or interface,
can be provided in a process for spinning a hollow fiber
membrane. Moreover, in spinning procedures, the polymer
solution must be of sufficient viscosity to provide a
~elf-sùpporting extrudate prior to and during coagulation,
and the coagulation must be quickly effected after
extrusion such that the hollow fiber membrane is not
adversely affected.
Processes for the formation of integral anisotropic
membranes must not only meet the criteria for forming
~5 integral anisotropic hollow fiber membranes but also
must be compatible with hollow fiber spinning capabilities.
Hence, many constraints are placed upon the techniques
available to produce integral anisotropic hollow fiber
membranes. Commonly, in hollow fiber membrane spinning
procedures, a solution of the polymer to form the hollow
fiber membrane in a solvent is extruded through a
spinnerette suitable for forming a hollow fiber structure,
and a gas or liquid is maintained within the bore of the
hollow fiber extrudate such that the hollow fiber
configuration can be maintained. The hollow fiber
extrudate must quic~ly be coagulated, e.g., by contact
with the non-solvent for the polymer, such that the hollow
fiber configuration can be maintained. The hollow fiber
114~510
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spinning process contains many variables which may affect
the structure, or morphology, of the hollow fiber
membrane such as the conditions of the polymer solution
; when extruded from the spinnerette, the nature of the
fluid maintained in the bore of the hollow fiber membrane
- extrudate, the environment to which the exterior of the
hollow fiber extrudate is subjected, the rapidity of
coagulation of the po~ymer in the hollow fiber extrudate,
and the like. Since the hollow fiber forming process is
one of coagulation of polymer from a polymer solution, the
nature of the solvent for the polymer may be highly
influential in determining the morphology of the hollow
fiber mem~rane and its separation properties,
The solvent must possess numerous properties in order
to be suitable for orming anisotropic mem~ranes
Cespecially anisotropic hollow fiber membranes). For
example, the solvent (or a liquid carrier containing
solvent) must be capa~le of dissolving the polymer for
forming the hollow fiber membrane but yet permit the
polymer to readily coagulate to form the anisotropic
structure, Furthermore, when hollow fiber membranes are
desired, the solvent Cor a liquid carrier containing
solvent) should enable polymer solution to be prepared
having suitable viscosities for hollow fiber membrane
formation, and advantageously, these viscosities can be
obtained without using excessively high concentrations
of polymer in the solution. Since advantageous hollow
fiber membranes are often obtained when the extrudate
incurs a significant temperature drop upon exiting the
spinnerette, the liquid carrier should enable suitable
viscosities for hollow fiber formation to be provided at
ele~ated temperatures which elevated temperatures
facilitate achieving the significant temperature drop.
The solvent or any other components of the liquid carrier
should not be subject to degradation at such elevated
temperatures. The solvent should be miscible with
non-solvent used to assist in coagulating the polymer and
should be capable of being removed, e.g., by washing,
4~510
07_0433
from the coagulated structure such that the membrane is
not unduly plasticized, and thereby weakened, by the
solvent. Moreover, the solvent Cor a liquid carrier
containing solvent) should not exhibit excessive heats
of dilution in the non-solvent used to assist in
coagulating the polymer.
In order for a procedure to be attractive for the
-- production of commercial quantities of membranes, it is
- also desired that the procedure be safe and economical.
Thus, the solvent should not be unduly toxic, and
advantageously, the solvent exhibits a very low vapor
pressure to minimize risk of inhalation andlor air
pollution. Moreover, a sol~ent having a very low vapor
pressur~. may also minimize the risk of explosion and fire.
Furthermore, waste materials from the spinning process
should be able to be economically and safely discarded or
recycled.
Si.nce the solvent is only one component used in the
splnning procedure, other components such as fluid within
the bore of the hollow fiber extrudate, non-solvent to
assist in effecting coagulation, washing fluids to remove
solvent from hollow fiber membranes, and the like should
also be economical and safe. Heretofore proposals have
been made to use, e.g., gasoline, kerosene or other
hydrocarbonaceous materials in the spinning procedure
either as coagulants or to assist in drying such as
disclosed by Arasaka, et al., in U. S, Patent No. 4,127,625.
Such materials clearly pose toxicity and fire risks as
well as disposal problems. Moreover, in the quantities
required to effect, e.g., coagulation, washing, etc., the
expense of the hydrocarbonaceous materials could be a
factor in the economics of the spinning process.
Accordingly, it is desired to use highly safe, readily
available materials, such as water, wherever possible in
the spinning process, especially as non-solvent to assist
in effecting coagulation and in washing to remove solvent
fro~ the hollow fiber membrane, The ability to use water,
or course, will depend to a large extent upon the
1~41510
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properties o~ the solvent with respect to water, i.e.,
solubility in water, heat of dilution in water, stability
in water, and the like.
By this invention methods are provided for preparing
anisotropic membranes, which methods utilize certain
ad~antage~us solvents. The methods of this invention can
provide anisotropic membranes characterized by having a
very thin separating layer having relatively few pores on
an underlying region having an open, cellular structure
which offers little resistance to fluid flow. These
membranes may be particularly attractive for gas
separations. Moreover, po-lymers which may be utilized in
accordance with this invention to provide membranes
include polysulfones which exhibit desirable strength
and chemical resistance as well as desirable permeation
properties in terms of permeate 1ux and selectivity.
Advantageously, the methods of this invention can utilize
water as a non-solvent to assist in effecting the coagulation
of the polymer and to wash solvent from the membranes.
Moreover, the solventq have low vapor pressures, do not
pose an undue risk of explosion or fire, and may be
economically and safely discarded or recycled.
In accordance with this invention, an anisotropic
membrane is prepared from a solution of membrane-forming
polymer in a liquid carrier comprising an N-acylated
heterocyclic solvent which can be represented by the
structure:
X N---C--R
30 ~
wherein X is -CH2-, -N(R')- or -O-; ~ is hydrogen, methyl,
or ethyl; and R' is hydrogen or methyl. The polymer
solution is provided in the form of a precursor and is
then coagulated in a liquid coagulant comprising water
to form the anisotropic membrane, The anisotropic
membrane may be a fi~m or, preferably, a hollow fiber.
Typical N-acylated heterocyclic solvents include l-formyl-
piperidine, l-acetylpiperidine> l-formylmorpholine, and
~141Sl~
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l-acetylmorpholine. A preferred N-acylated heterocyclic
solvent comprises l-formylpiperidine. Reilly Tar &
Chemical Corporation, in its brochure entitled "l-Formyl-
piperidine", reports that l-formylpiperidine has a boiling
point of about 222C, a vapor pressure of 0.111 millimeters
of mercury at 25C, a flash point of about 102C, and an
LDso (oral-rats) of about 0.88 grams per kilogram of body
weight.
In the methods of the aspect of this invention
pertaining to makiug hollow fiber membranes the polymer
solution is at a sufficient temperature to substantially
maintain the fiber-forming polymer in solution and to
provide the polymer solution at a iber-formIng viscosity
prior to the extrusion of the hollow fiber precursor.
The polymer solution is extruded through an annular
spinnerette to form a hollow fiber precursor. A fluid is
in;ected into the bore of the hollow fiber precursor as
it i9 being extruded from the spinnerette at a rate
sufficient to maintain the bore of the hollow fiber
precursor open. The injection fluid is preferably highly
miscible with the liquid carrier and often, therefore,
comprises water. The hollow fiber precursor is then
contacted with the liquid coagulant which is a non-solvent
fo~ the polymer in the polymer solution. The liquid
coagulant is preferably highly miscible with the liquid
carrier and the injection fluid. Usually the temperature
of the liquid coagulant is sufficiently low that the
polymer solution at that temperature is extremely viscous
ant may even be a gel. The contact of the hollow fiber
precursor with the liquid coagulant is for sufficient
duration to substantially completely coagulate the polymer
in the hollow fiber precursor under the conditions of the
liquid coagulant and thereby provide a hollow fiber. The
hollow fiber is then washed, i.e., contacted, with a
non-solvent comprising water for the polymer which is
miscible with the liquid carrier to reduce the content
of the liquid carrier in the hollow fiber to less than
about 5 weight percent based on the weight or the polymer
--` 1141S10
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in the hollow fiber. The washed hollow fiber may then be
dried at a temperature at which the selectivity or flu~
exhibited by the hollow fi~er is not unduly adversely
affected.
In an aspect of this invention the hollow fiber
membranes preferably have a relatively circular cross-
section with circular, concentric bores. In many instances,
the hollow fiber membrane can be circular and concentric
within the tolerances of the machining required to ma~e
suitable spinnerettes. The outside diameter of the
hollow fi~er membrane may vary widely, e.g., from about
100 or 150 to 1000 or more microns. Frequently, the
outside diameter of the hollow fiber membrane is about
200 or 300 to 300 microns.
The ratio of thickness of the wall to outside diameter
of the hollow fiber membrane may also vary widely depending
upon the required collapse pressure which must be exhibited
for a given separation operation. Since the cellular
support i9 open, an increase in wall thickness may not
result in an undue reduction in flux through the membrane
wall. Typical ratios of thickness to outside diameter
a~e about 0.1 or 0,15 to about 0.45, most often about
0.15 to 0.4.
The void volume of the hollow fiber membranes, i.e.,
regions within the wall o t~e hollow fiber membrane
vacant of the material of the hollow fiber membrane is
substantial. The void volume can be determined by a
density comparison with a volume o~ the bulk polymer (or
other material of the hollow fiber) which would correspond
to a hollaw fiber of the same superficial gross physical
dimensions and configurations as the wall of the hollow
fiber membrane. Since the cellular support of the hollow
fibers is open, relatively low void volumes for
anisotropic hollow fiber membranes ca~ be achieved without
unduly low permeate flu~ occurring. Often, the void
volume is at least aboùt 30 to 35, e.g., at least about
40, say, greater t~an about 45 or S0, volume percent,
and may range up to 65 ~r 70 or more volume percent,
4~ 5i~
_9_ 07-0433
The liquid carrier may consist essentially of the
solvent or the`solvent may be admixed with one or more
other components to provide the liquid carrier. The
other components ~ay be solid or liquid at ro~m
temperature ~ut are dissolved when in the Liquid carrier.
In many ins~ances, especially when the precursor is
exposed to the atmosphere prior to coagulation of the
polymer, it is preferred that t~e liquid carrier
Cincluding any diluent) be relatively non-volatile, e.g.,
the liquid carrier advantageously has an essential absence
of a component (solvent or diluent) having a vapor
pressure above about 0.8, say, above 0.6, atmosphere at
the temperature of the polymer solution at extrusion.
r If, however, a more volatile component in the liquid
carrier i9 used, and the precursor is exposed to an
atmosphere prior to contacting the liquid coagulant,
the atmosphere to which the precursor is exposed, may
beneficially contain substantial amounts of such a more
volatile component to retard the loss of the component
2~ rom the precursor. Gaseous components are generally
avoided but may be of use especially when the precursor
i9 immediately contacted with the liquid coagulant. The
components are preferably compatible with the solvent and
the liquid coagulant. Often the other components are
highly soluble in the N-acylated heterocyclic solvent,
e.g., at least about 50, preferably at least about 100,
part3 by weight can be dissolved in 100 parts by weight
of the N-acylated heterocyclic solvent at room temperature
(about 25C), and the other components are preferably
miscible with the N-acylated heterocyclic solvent and
the liquid coagulant in all proportions. C~mponents
other than the N-acylated heterocyclic solvents which
may be useful in the liquid carrier include solvents,
such as dimethylacetamide, dimethylformamide, N-methyl-
pyrrolidone, dimethylsulfoxide, etc,; viscosity modifiers,such as isopropyl amine; surfactants; plasticizers; and
the like. A particularly useful component for the liquid
carrier is a diluent which may be a solvent or non-solvent
for the polymer. Preferably, the N-acylated heterocyclic
:
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114~Sl(~ -
solvent comprises at least about 50, say, at least a~out
60, weight percent of the liquid carrier,
The use of diluents can provide particularly
attractive li~uid carriers such that certain solvent
activities with ~he individual polymer molecules are
exhibited such that frequently, the polymer can be
coagulated or gelled from the polymer solution with little,
if any, change in energy, The ~ehavior of polymer
molecules in solution is theorized to be influenced by
not only the interaction among the polymer molecules,
but also b~ the effect of the solvent (and liquid carrier)
on the polymer molecule.
In accordance with the theory described by Flory,
Principles of Polymer Chemistry (1953),
at Chanter XIV~, ~a~es 595 to 634, in a ~iven
~olvent-polymer system at a given temperature, the
molecular dimension of the polymer may be unperturbed by
intramolecular~interactions. As the temperature i~creases
from this given temperature Ctheta temperature), the polymer
swells in the solution, The greater the "swelling" of the
polymer molecule at a reference temperature, the better
the solvent is for the~polymer at the reference temperature.
If the temperature is decreased from the theta temperature,
a polymer molecule of infinite molecular weight would gel~
or precipitate, i.e., the intramolecular forces of ~the
polymer molecule are greater than the forces between the
solvent and polymer molecule.
A meaningful evaluation of a liquid carrier polymer
systEm can ~e made by viscosity measurements of a very
dilute solution of the polymer in the liquid carrier to
determine the intrinsic viscosity ~the capacity of a
polymer to enhance the VisCQSity of a solution) ~e,g,, at
approximately 25C for the sake of consistency although
other temperatures so long as consistent, can be used)
and the intrinsic viscosity of the pol~mer at the theta
te~perature cintrinsic theta viscosity). The ratio of
the intrinsic viscosity to the intrinsic theta viscosity
Chereafter "intrinsic viscosities ratio"2 is indicative
.
r~ f~ ,~
11 9f~510 07 4
of the effect of the solvent (liquid carrier) on the
polymer molecules. The intrinsic viscosity for a given
l~quid carrier polymer system is obtained by determining
the viscosity of polymer solution Cn) at several various
S dllute polymer concentrations, The viscosity of the
liquid carrier CnO) is also deter~ined at the same
temperature. The ratio of n:~O is the relative viscosity
and a plot of the natural logarithm of the relative
vl~cosity divided by the polymer concentration versus
the polymer concentration when extr2polated to zero
polymer concentration provides the intrinsic viscosity.
Con~eniently, the dilute polymer solutions ~mployed are
seLected to provide relative viscosities of from about
1,1 to 2.5. In order to determine the intrinsic
viscosity of the polymer at the theta temperature,
several techniques may be employed such as disclosed by
FLory, supra, at pages 612 to 622. For instance, the
intrinsic viscosity of the polymer in a theta solvent
at the theta temperature can be determined by viscosity
~ ~easurements as set forth a~ove. Theta solvents and theta
te~pera~ures for many nol~ners c~n be found in the literature,
e.~., see ,o- instance srancl~up, e~ al., ~olymer Handbook,
Second Edi~ion (1975), pa~es IV-157 to I~l-173. I~ necessary,
theta solvents and temperatures can be determined by any
of the techniques set forth by Brandrup, et al,, at
pages IV-157 to IV-159, The temperature of the polymer
solution during the viscosity determination must be
precisely maintained as close as possible to the theta
temperature since the intrinsic viscosity of many polymer
solutions changes rapidly at temperatures close to the
theta temperature, Alternatively the intrinsic theta
viscosity can be estimated by the use of a light
scattering technique to determine the second virial
coefficients of the polymer in a solvent at different
temperatures. A suitable procedure is disclosed by Kaye
in "Low Angle Laser Light Scattering", Analv~ical Chemist-_,
45 Cl973) pages 221 to 225, and involves the determination
;~ ' ' ' .
_ _ _ _ _ _ _ _ _
~14~510
-12- 07-0433
of the amount of light scattering of a lassr beam passe~
through the polymer solution at several temperatures. See
for instance, Stacy, Li~ht Scattering in Physical
Chemistry (1956) pages 117 et seq., for a description of
the relationship between light scattering and the second
virial coefficient. Care should be taken to remove any
solids from the polymer solution, e,g., by filtration,
which can affect the light scattering data. The intrinsic
viscosity of the polymer is obtained at each of the
t~mperatures at which the second virlal coefficient is
determined, and a plot of the intrinsic viscosities and
second virial coefficients can be prepared and extrapolated
to the point at which the second virial coefficient is
zero. The intrinsic viscosity at this point is the
intrinsic theta ~iscosity, The met~od for determining the
intrinsic theta viscosity is ~n approximation; however,
such an approximation may still be useful in providing a
suitable polymer-liquid carrier system for use in
accordance with this invention to make a hollow fiber
membrane fr~m a polymer, Generally suitable polymers in
the liquid carriers in accordance with this invention will
provide dilute solutions exh~biting an "intrinsic
viscosities ratio" of about 1,05 to 1,7, say, about 1,05
to 1,5, and most frequently about 1,1 to 1.5.
The second virial coefficient of the liquid carrier/
polymer system may similaxly be useful in determining
suitable liquid carrier/polymer systems. The second
virial coefficient exhibited by many useful liquid
carrier/polymer sy~tems often is below about 15, e.g,,
below a~out 12, say below about 10, mol-cm3/gm2 (times
104~ Cat 25C). The second virial coeficient for the
liquid carrier/polymer systems is frequently above about
0.5, say, a~ove 1, and sometimes above 3, for instance,
between about 3 and 8, mol-cm3/gm2 Ctimes 104) ~at 25).
Another useful method for determining liquid
carrier/polymer systems which may be ad~antageous for
providing hollow fiber mem~ranes in accordance with the
methods of this Lnvention is the interaction par~meter
. ,
-13-
described by Blanks, et al., in "Solubility of Styrene-
Acrylonitril~ Copolymers", Structure-Solubility
Relationships in Polymers, edited by Harris, et al., (1977)
pages 111 to 122. The interaction ~arameter (A12) can be
defined a~ follows:
~2 .(V~/RT) [(~d, ~d, ) + 0,25[~p ~ ~p ) + (~h ~ ~h )
where Vm is the molar volume of the solvent in cubic
ee~imeters per mole; R is the gas constant (about 12
cal¦K mol); T is temperature, K; ~d and ~d are the
(London) dispersive force component of the solubility
parameter, (cal/cc)~, of the polymer and solvent
respectively, ~p p and ~p are the polar bonding terms
of the solubility parameter, Ccal/cc)~, of the polymer
and solvent respectively; and ~h p and ~h are the
15~ hydrogen bonding terms of the solubility parameter,
(cal/cc)~, of the polymer and solvent respectively. The
total solubility parameter (~T) can be defined as
~T2 - ~d2 + ~p + ~h
Solubility parameters are conveniently employed to
characterlze liqùid carriers and solutes, and values for
the solubility parameters and components can often be
found in the literature, for instance, see Shaw, "Studies
of Polymer-Polymer Solubility Uslng a Two-Dimensional
Solubility Parameter Approach", J~ of Applied Polymer.
Science, 18, pp. 44g to 472 ¢1974), and Brandrup, et al.,
The Polymer Handbook, Second Edition, pages IV-337 to
IV-359 ~who also descrlbe experimental techniques for
obtaining solubility parameters and components). The
components). The components of the solubility parameter
which are used to calculate the interaction parameter for
a liquid carrier comprising more than one component can
be estimated on a volume raction average basis. This
estimation is an approximation since in many instances
. , .
_ . ...
`` ~14~510
-14- 07-0433
the volume of the components of liquid carrier are not
additive. Frequently the interaction parameter at 20
to 30C of the polymer and liquid carrier is less than
about O.5, say, less than about O.3. The interaction
parameter generally is at least about O.Ol,preferably,
at least about 0.02, e.g., at least about 0.05.
Also, the liquid carrier advantageously has a
solubility parameter approximately equal to (i.e., within
5 or lO percent of) or above the solubility par~me~er of
the polymer. In many instances, at least one, and sometimes
both, the polar bonding term and hydrogen bonding term of
the solubiLity parameter are greater (i.e., no more than)
0.5, say, greater than 0.3 (cal/cc)~, below the
corresponding term for the polymer. Advantageous membranes
can be m de from polymer solutions in which the addition
of Liquid coagulant, even in very small amounts, does not
substantially improve the ability of the solvent to
dissolve the polymer. Often, the polar bonding term and
hydrogen bonding term of the solubility parameter of the
liquid carrier are each greater than the corresponding
terms of the solubility parametér of the polymer.
Often, the surface tension of the liquid carrier i9
within about lO, say, about 5 or 7, dynes per centimeter,
of the surface tension of the polymer at room temperature
C25C). Generally, if the liquid carrier/polymer system,
when in a dilute polymer solution, e~hibits a suitable
"intrinsic viscosities ratio", second virial coefficient,
and interaction parameter, the liquid carrier also
exhibits a surface tension approximating the surface
tension of the polymer, Frequently, particularly useful
liquid carriers OEhibit a surface tension at room
temperature C25C) less than about 3 or 5 dynes per
centimeter abo~e the surface tension of the polymer.
Surface tensions for polymers can be OEperLmentally
determined by any conventional procedure, and one such
procedure is described by Tanny in J, of Applied PolYmer
Science, Volume 18, pages 21~3 to 2154 (1974).
,
. .
-` 114~510
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Often liquid carriers used in the method of this
invention contain a diluent which is a non-solvent for
the polymer. Non-solvents are generally characterized by
exhibiting little capability of dissolving polymer, e.g.,
the polymer solubility is less than about lOj say, less
than 2, often less than about 0,5, grams per 100 milli-
liters of non-solvent, The non-solvent preferably exhibits
little, if any, swelling action on the polymer. The
non-solvent, if added in a sufficient amount, is usually
capable of resulting in a phase separation of the polymer
solution. Preferably, the non-solvent is not added in
an amount such that the polymer solution is unduly
unstable at the processing conditions prior to forming
the precursor. Frequently, the amount of non-solvent in
the liquid carrier is at least about 2, e.g., at least
about 5, weight parts per 100 weight parts of the liquid
carrier.
Suitable non-solvents can include the liquid
coagulant or one or more components of the liquid
coagulant. Some desirable solvents can maintain the
polymer in solution in the presence of at least about 5,
say, at least about 10, weight percent of the liquid
coagulant based on the weight of the solvent ~e,g., at
25C). Preferably, however, the addition of relatively
small quantities of liquid coagulant to a solution of the
polymer in the liquid carrier will result in phase
separation or gelling of the polymer~solution.
Typical diluents, including non-solvents, include
formamide, acetamide, ethylene glycol, water, trimethyl-
amine, triethylamine, isopropylamine, isopropanol, methanol,
nitromethane, 2-pyrrolidone; acetic acid, formic acid,
aqueous ammonia, methylethylketone, acetone, glycerol
and the like. Low molecular weight inorganic salts such
as lithium chloride, lithium bromide, zinc chloride,
magnesium perchlorate, lithium nitrate, and the liXe
may also be useful in the liquid carrier.
A sufficient amount of polymer is contained in the
polymer solution to enable the precursor to be formed.
114~510~
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Hence, when preparing a hollow fiber m~mbrane, the polymer
concentration is sufficient that the polymer solution is
at a fiber-forming viscosity at the temperature of the
polymer solution when formed ~i,e., extruded) into a
hollow fiber precursor. Unduly low viscosities can result
in the breakage of the hollow fiber precursor and the
inability to maintain the desired hollow fiber configuration.
High viscosities are desirable, ~ut excessively high
viscosities may be undesirable due to the pressures
required to extrude the polymer solution, Frequently the
visco~ity of the polymer solution being extruded is at
least about 5000, often at least a~out 10,000, centipoises
and may be as high as 500,000 or 1,000,000 centipoises at
the temperature of the extrusion, Many attractive hollow
fiber membranes are prepared utilizing polymer solution
viscosities at t~e temperature of extrusion in the range
of about 10,000 to 500,000 centipoises, In some instances,
sufficient polymer may be contained in the polymer
solution that at the temperature of the liquid coagulant,
the polymer ~olution becomes a substantially non-flowing
structure, for instance, becomes extremely viscous or
undergoes a physical change, e.g,, to fcrm a gel, (i.e.,
an elastic structure in which at least some of the polymer
is not soluble in the liquid carrier and liquid carrier
i9 entrapped in the interstices) or to result in phase
8 eparation.
The polymer concentration of the polymer solution is
conveniently sufficiently high in order to ensure that
the membrane contains suficient polymer to provide the
desired high strength to the membrane. If the polymer
concentration of the polymer solution is too low, large
voids, including large cells and macrovoids, may occur
in the membrane wall, thereby resulting in a low strength
wall structure, At higher polymer concentrations the
resulting hollow 'iber wall is generally more dense and
macrovoids are generally more infrequent, hence the
hollow fiber membrane can exhibit greater strength.
Macrovoids are large voids having a major dimension
-`` ll~S~O~,`
-17- 07-0433
greater than about 3 microns, Preferably, a sufficiently
high polymer concentration is ~mployed such that under
the coagulation conditions, few, if any, macrovoids are
formed. Since this invention advantageously can provide
a~ ope~, cellular structure in the membrane.wall, the
increase in polymer density can be achieved with little,
if any, reduction in flux through the m~trix of the
membrane wall. The maximum polymer concentrations which
ca~ be used in a polymer solution is gençrally determined
o~ a practical ~asis by the capability of conventional
apparatus to form membranes from highly viscous polymer
solutions, The maximum, preferred concentrations of
pol~mer in the polymer solution will also depend on the
nature of the polymer and of the liquid carrier. For
instance, with lower molecular weight polymers, higher
polymer concentrations can be more desirably utilized
than with higher molecular weight polymers. Frequently,
the polymer concentration is at least about 25 weight
percent of the polymer solution. Polymer concentrations
as high as 45 or S0 weight percent may be useful in some
situations. Polymer concentrations of about 28 or 30 to
38 or 40 weight percent are most often desired,
Frequently, the viscosities of the polymer solutions
used in accordance with this invention are of relatively
high activation energies, An activation energy is a
relationship between temperature and viscosity, and this
relationship has been characterized as:
n = A eE/RT
where n is the viscosity of the polymer solution, E is
the activation energy, R is the gas constant Capproximately
2 cal/K, mole) and T is the temperature CK). See Kunst,
et al,, Fifth International Symposium on Fresh Water rrom
the Sea, Vol. 4 C1976) .
The slope of a plot of the logarithm of the viscosity
(poise) to the reciprocal absolute temperature (K-l), is
,
~ s~o
-18- 07-0433
- generally linear, and from that slope, the activation
energy (cal/mole) can be appro~imated, Often, the
activation energy is at least about 8 kcal/mole, say,
about 8.5 to 15, e.g., a~out 9 to 12, most frequently
about 9.5 to 12, kcal/mole, Usually, with desirable
liquid carriers containing a non-solvent as a diluent,
the activation energy increases with increased
concentration of the non-solvent.
The polymer solution can be prepared in any convenient
manner, for instance, the liquid carrier can be added to
the polymer, the polymer can ~e added to the liquid
carrier, or the polymer and the liquid carrier can be
simultaneously com~ined. The liquid carrier, of course,
if comprising more than one component, can be admixed with
the polymer on a component-by-component ~asis. For
instance, if the liquid carrier comprises a solvent and
a non-solvent, the polymer may advantageously be admixed
with the solvent prior to the addition of the non-solvent.
When the non-solvent is added subsequent to dissolving the
polymer in the solvent, generally the non-solvent is added
910wly such that the localized zones of increased non-
sol~ent concentrations are minlmized to avoid coagulation,
or precipitation, of the polymer at such localized zones.
Elevated temperatures may be utilized to facilitate the
mixing of the polymer and liquid carrier, The temperature
however, should not be so high as to deleteriously affect
any of the components of the solution being formed. The
time required to effect mixing to provide a polymer solution
can vary widely depending upon the rate of solution of
the components, the temperature, the efficiency of the
mixing apparatus, the viscosity of the polymer solution
being prepared, and the like. Desira~ly, the mixing of
the polymer solution should continue until a substantially
uniform composition exists throughout the polymer solution.
Any suitable mixing equipment may find application in
preparing polymer solutions suitable for use in accordance
with this invention. Preferably, the mixing equipment
does not induce undue air entrapment in the polymer
1~415iO
- 19- 07 -0433
solution in order to facilitate any su~sequent degassing
of the polymer solution. Often components of the polymer
solution have an appreciable volatility under the
conditions of mixing, These volatile components may
present undue risk in terms of fire and health hazards,
and the loss of volatilized components will also alter
- the composition of the polyme'r solution. Accordingly,
in most instances the mixing of the polymer and liquid
carrier is conducted in an essentially sealed container.
A~ inert atmosphere may be provided in such sealed
containers, It is possi~le that components for the polymer
solution may be adversely affected when exposed, to
atmospheric conditions ~e,g,, ambient air), The total
pressure of the atmosphere in the mixing co~tainer is
preferably relatively low in order to minimize any
dissolving or entrapment of the inert atmosphere in the
polymer solution during formation of the polymer solution.
In most instances, pressures below about 2 or 3 atmospheres
absolute are used.
Frequently the polymer solution contains entrapped or
aissolved gases, which gases may result in the formation
of anomalie3, e.g., m2cro~0ids, in the membranes. Thus,
it is generally desirable to sub~ect the polymer solution
to degaqsing operations. Preferably, the polymer solution
is substantially free of entrapped or dissolved gases
prior to extruding the hollow fiber precursor. Any
suitable degassing apparatus and conditions may be useful
in effecting ~he desired degassing, For ex~mple, adequate
degassing may be achieved by holding the polymer solution
in a closed vessel for a time qufficient to allow the
entrapped or dissolved gases to escape the polymer
solution. Conventional degassing equipment such as "J"
tubes, centrifugal degassers, ultrasonic degassers, and
the like, may find application in degassing the polymer
solution. Since many of the polymer solutions useful
in accordance with this invention are of relatively
high viscosity, the polymer solution in the degasser may
be at an Plevated temperature in order to reduce its
510
-20- 07-0433
viscosity and facilitate release of the entrapped or
dissolved gases. The ele~ated temperatures to which the
polymer solution may be subjected and the times for which
the polymer solution is at such elevated temperatures,
should not be so excessive as to result in deleterious
effects to the polymer solution or its components. The
degassing may be conducted at relatively low absolute
pressures (i,e,, vacuum~ or at such higher pressures as
to prevent e.g., undue volatilization of any of the
components of the polymer solution. The pressure should
not be so high as to result in an undue dissolving or
redissolving of gases from the atmosphere in the degasser.
In general, the degassing is preferably conducted at an
absolute pressure below about 2 atmospheres, and most
often the degassing is conducted at a su~atmospheric
pressure, The degassing may be conducted in a sealed
vessel. An inert atmosphere may be used during the
degassing operation.
The degassed polymer solution can then be used for
forming the membrane precursor. In transporting the
polymer solution, it is preferred that the piping and
pumping apparatus be designed such that gas ~e.g., air~
does not leak into the transport system and enter the
polymer solution. The polymer solution may contain
solit impurities, e.g., dust, polymeric oligomers, and
the like, which may ad~ersely affect the mem~rane. Often,
therefore, the polymer solution is filtered prior to
entering the spinnerette. Prefera~ly, the filtering is
sufficient to remove substantially all solid particles
8reater than about, say, 50 micron in ma~or dimension,
and often substantially all particles greater than about
0.5 micron in greatest dimension and removed by filtering.
Elevated temperatures may be used to facilitate any
transport and filtering of the polymer solution. Such
elevated temperatures however should not be so high as
to result in any deleterious effects to the polymer
solution or its components.
- 114~510
-21- 07-0433
Frequently, water comprises a major amount of the
liquid coagulant sufficient to provide a suitable strength
non-solvent to effect the polymer coagulation. For
instance, the liquid coagulant may comprise at least about
50, say, at least about 75, and most often at least about
85, weight percent water. Advantageously the liquid
coagulant does not cause any appreciable swelling of the
polymer, Since water often comprises a major amount of
the liquid coagulant, the polymer preferably exhibits
little, if any, swelling in water. Desirably, the
composition of the liquid coagulant is such that the hea~
of dilution of the liquid carrier in the liquid coagulant
is greater than about -3,5, say, greater than about -3,
e.g., greater than about -2.5, and frequently, greater
than about -2 kilocalories per mole. The absolute value
of the heat of dilution of the liquid carrier in the
liquid coagulant is often less than about 3 kilocalories
per mole, In many instances, the heat of dilution is
within the range of about -0.5 to -2 kilocalories per
mole. The heat of dilution can be determined using any
conventional procedure and generally is determined at
about 25C.
The liquid coagulant, particularly when making hollow
fiber membranes on a continuous basis, will contain liquid
carrier which is removed from the precursor. The liquid
carrier may detract from the non-solvent strength of the
liquid coagulant. ~he liquid coagulant is therefore
desirably replaced with fresh, or recycled, liquid
coagulant containing little, if any, liquid carrier.
Frequently, the concentration of the liquid carrier in
the liquid coagulant is less than about 10, prefera~ly
less than about 5, say, less than about 2, weight percent.
The liquid coagulant may contain additional components
such as surfactants, materials which increase or decrease
the non-solvent strength of the liquid coagulant with
respect to the polymer, materials which enhance the
solubility o components of the liquid carrier in the
liquid coagulant, materials which reduce the heat of
510
-22- 07-0433
dilution of the liquid carrier in the liquid coagulant,
freezing point depressors, and the like. Useful
additional components for the liquid coagulants which may
find application in making the membranes include low
molecular weight alcohols such as methanol, isopropanol,
etc., salts such as sodium chloride, sodium nitrate,
lithium chloride, etc., organic acids such as formic
acid, acetic acid, etc., and the like.
The liquid coagulant is desirably maintained at a
temperature at which the polymer solution is substantially
non-flowing. While suitable temperatures are generally
l~w, some polymer solutions may be su~stantially non-
flowing at room temperature or above. Hence, liquid
coagulant temperatures of up to about 90C or higher may
find application. Often, however, the liquid coagulant
temperature is below about 35C, and highly desirable
membranes are frequently produced using liquid coagulant
temperatures below about 20C, say, below a~out 10C.
Most often, for the sake of convenience, its temperature
~ i9 abo~e about 0C, e.g., about 0 to 10C. However, with
suitable refrigeration equipment and the presence of
freezing point depressors in the liquid coagulant,
temperatures of -15C and below may be achieved~
The residence time of the precursor in the liquid
coagulant should ~e sufficient that the amount of
coagulation at the temperature of the liquid coagulant
provides adequate strength to the membrane for further
processing. Frequently, it is desired that coagulation
of the polymer in the precursor occur within a few
seconds such that the equipment sizes for the coagulation
step are not unduly large. Since the exterior of, e.g.,
the hollow fiber precursor directly contacts the.liquid
coagulant, it is generally almost instantaneously
coagulated. In some instances, essentially complete
coagulation of the polymer in the precursor under the
conditions of the liquid coagulant may almost
instantaneously occur. More freauently, the precursor
exhibits an observable transition in the liquid coagulant
from a clear or translucent structure to an opaaue
~ , .
- ~ 114~510
-23- 07-0433
structure. This transition may be gradual, and sometimes
t~e progress of the transition can be observed. The time
for this transition under the conditions of the liquid
coagulant can vary widely, but in many instances is
at least about 0.001, for example, about 0.01 to 1, say,
about 0,02 to 0.5, seconds.
Generally, even after coagulation of the precursor,
the membrane still contains substant~al amounts of liquid
carrier which can adversely affect its strength properties
and may even enable coagulated polymer to be redissolved.
Most con~eniently, therefore, the membrane is subjected to
at least one washing step to further remove liquid carrier.
Often, the washing of the membrane is initiated uh~der
substantially the same contitions Ce.g,. temperature)
that the coagulation occurred, The temperatures employed
for washing are often based on the strength of the liquid
carrier-containing membrane, the useful range of
temperatures to which the washing liquid can be subjected,
and co~enience of obtaining the washing liquid at such
temperatures. In general, temperatures should be avoided
during washing which could result in undesirable annealing
o the membrane surface. Often the temperature of washing
ranges from about 0 to 50C preferably about 0 to 35C.
Conveniently, especially if the liquid carrier and liquid
coagulant are miscible wlth water, water is employed as
the washing 1uid, The washing fluid may contain
additives to, e.g., enhance removal of the liquid carrier.
For instance, if water is the washing fluid, then a water
misci~le organic material Ce.g., methanol, isopropanol,
etc.) may assist in facilitating the removal of the
liquid carrier.
Often the liquid carrier content of the membrane can
readily be reduced to say less than about 20 weight percent
of liquid carrier Ce.g,, by passing the ~ollow fiber through
a washing liquid for 2 to 5 minutes~. However, the
reduction of the liquid carrier content of the membrane
to desirably low levels, e~g., less than about 5, and
sometimes less than about 2, weight percent liquid
. .
-24- 07-0433
carrier, may require relatively long periods of additional
washing. This additional washing is of~en at least about
3 hours, and may range up to 20 or more days. The
additional washing may be conducted by continuously passing
washing liquid having little, if any, liquid carrier over
the membrane (rinsing) or by soaking, or storage, in
washing liquid. Generally, a combination of rinsing and
storage is employed to remove the liquid carrier from the
membrane. When the membranes are stored in washing liquid,
a periodic replacement of washing liquid may be advantageous
if the concentration of liquid carrier in the washing
liquid becomes undesira~ly high. Although the membranes
can be stored in the washing liquid for periods longer than
20 days, usually little additional amounts of liquid
carrier are removed from the mem~ranes after such long
storage periods. Most frequently, t~e washing of the
membranes continues unt~l the liquid carrier content of
the membrane is less than about 4 or 5 we~g~t percent of
the membrane.
The membranes, after washing, are dried to remove the
washing liquid. The presence of liquid in the anisotropic
m~mbranes can significantly reduce the flux of moieties,
e.g., gases, permeating the membrane and therefore is
generally not desired. Suitable drying can be accomplished
by exposure to a gaseous atmosphere. Air is usually a
suitable gaseous atmosphere. Useful drying conditions can
vary widely. For instance, suitable drying conditions may
include temperatures ranging from -15C or below to 90C
or more and relative humidities ranging from about 0 to 95,
say, about 5 to 60 percent. Frequently, the temperature
ranges from about 0 to 80C with absolute humidities less
than about 20 or 30, say, about 5 to 15, grams per cubic
meter. In many instances, drying membranes under ambient
laboratory conditions, e.g., about 20 to 25C and 40 to 60
percent relative humidity, is acceptable.
Materials useful for preparing anisotropic membranes
may be organic polymers, or organic polymers mixed with
inorganics, e.g,, fillers, reinforcements, and the li~e.
f-~ r~
" ` ^114~i;0
-25- 07-0433
Thus, the method of this invention may find advantageous
applicability in the preparation of metallic hollow fiber
me~branes in accordance with the teachings of Dobo, et al.,
Unlted States Patent No. 4,175,153.
Sui-,able ~oly~ners are soluble in the
N-acylated heterocyclic solvents employed in accordance
wlth the method of this invention. Desirably, at least
about 50, preferably at least about 100, parts by weight
of polymer can be dissolved in 100 parts by weight of
the N-acylated heterocyclic solvent (at 25C~, and most
often, the N-acylated heterocyclic solvent and polymer are
miscible in all proportions. However, even though the
N-acylated heterocyclic solvent is a good solvent for the
polymer and is miscible with t~e liquid coagulant, the
polymer may not be acceptable to produce membra~es if the
solvent i9 not capa~le of being .eadily removed from the
membrane after coagulation, If t~e polymer retains the
solvent the polymeric structure of the membrane may be so
weakened as to result in undue compaction of the anisotropic
structure thereby resulting in an undesira~le reduction of
the permeability of the membrane to permeating moieties,
e,g., gases Moreover, if undue amounts of solvent are
~etained even after drying, t~e resulting membrane may not
have sufficient structural strength to withstand separation
condition~.
Frequently, the "intrinsic viscosities ratlo" for
suitable polymers in dilute soluion in the N-acylated
heterocycllc solvent is at least about 1.05, preferably a~
least about 1.3. Often the "intrinsic viscosities ratio"
is up to about 2, say, about 1.3 to 1 75, or even 1.5 to
1.75.
Frequently, the second virial coefficient for polymer
solutions containing a suitable pcl~ner in the N-acylated
heterocyclic solvent is below about 20, say, below about
15, mol-cm3/g2 (times 104) (at 25C). In many instances,
the second virial coefficient is at least about 0.5, say,
at least about 1, e.g., at least a~out 3, mol-cm3/g2
(timeg 104) (at 25C). Often the interaction parameter
~.,
~' ~ , .
. .
41 5i~
-26- 07~0433
for suitable polymers with the N-acylated heterocyclic
solvent at about 20 to 30C is less than about 0.5, say,
less than about 0.3, and most frequently at least about
0.005, say, at least a~out 0.01, and sometimes at least
about 0.02.
Generally, it is preferred that the polymer be
su~stantially non-crystalline, e,g., less than about 25
weight percent, and frequently less than about 1 or 2
weight percent, crystalline, since crystalline polymers
u~ually exhibit low permeability coefficients. However,
polymers containing substantial crystallinity may sometimes
~e desired. hlso, the po~ymer is often non-ionic to
facilitate the preparation of anisotropic membranes.
The polymeric material is preferably selected on the
basis of its separation capabilities, i.e., its ability to
provide the desired selectivity of separation at a suitable
permeate flux strength and chemical resistance, especially
with respect to the components of the fluid streams
intended to be treated using the membrane The polymer
should have a molecular weight sufficient for, e,g.,
hoLlow fiber formation, In general, for a given type of
pol~er, the longer the polymeric chain, the greater the
tensile strength exhi~ited by the polymer and the higher
the glass transition temperature of the polymer. These
increases in physical properties with increasing molecular
weight can often be accomplished with little, if any change
in the separation capabilities of the polymer, Thus, by
providing polymers of high molecular weight, the anisotropic
membrane can be made Rtronger and tolerate to a greater
extent the components of feed streams which can
deleteriously affect the polymer, However, increased
polymer molecular weights generally result in an increase
in viscosity of the polymer solutions for forming the
precursor Therefore, it is often desired to utilize a
polymer having a narrow distribution of molecular weights,
such that the polymer exhibits high strength properties
without an undue increase in viscosity, For instance, with
a broad molecular weight distri~ution, the polymer can
-'-` 114~510
-27- 07-0433
contain a significant portion of lower molecular weight
polymer molecules but provide a high solution viscosity
due to the presence of substantially higher molecular
weight polymer molecules. A convenient manner for
characterizing the molecular weight distribution of a
polymer is by the ratio of the weight average molecular
weight of the polymer to the number average lecular
weight of the polymer, The higher ratios ind~cate that
the molecular weig~t distribution is wider. Frequently,
this ratio is less than about 3.
The strength of the polymer should ~e sufficient to
prevent damage to the separating layer during expected
handling and transporting operations in making the
membrane, fabricating a permeator containing the membrane,
and installing and using the permeator. Also, the polymer
must exhibit sufficient strength such that the membrane can.
withstand gas separation operating conditions. The strength
of the polymer can be expressed in terms of, e.g., tensile
strength, Young's modulus, etc, While these mechanical
properties may each afect the overall strength of the
membrane, often an indication whether the polymer has
adequate strength can be obtained by a consideration of
the tensile strength o the polymer. Generally, suitable
polymers or membranes exhibit a tensile strength of at
least about 350, say, at least about 400, preferably at
least about 500, say, at least about 600 kilograms per
square centimeter. The tensile strength may be as high
as achievable, ~owever, few polymers which are advantageous
as membrane materials eghibit tensile strengths above about
2000 kilograms per square centimeter. The strength of a
polymer can be adversely affected by materials which, for
instance, dissolve, plasticize or sweLl the polymer. Often
many suitable polymers for membrane separations will be
dissolved, plasticized, swollen or otherwise adversely
affected by one or more materials. In some instances,
industrial streams, e,g., gas streams, to be treated using
membranes for separations contain one or more components
which can adversely affect t~e membrane material. By the
114~S10
-28- 07-0433
use of a membrane material which exhibits high strength,
a reduction in strength of the mem~rane due to the
presence of such delet~rious components in the feed
mixtures may be tolerated since the weaker membrane may
still withstand the separation operating conditions.
The selective permeation of many fluids, including
gases, in many polymers have been reported. In these
reported permeations the mechanism of t~e separation
apparently involves an interaction with the material of
the membrane. Hence, widely di~fering separation
capabilities can be achieved ~y the use of different
polymers. For instance, polyacrylonitrile permits nitrogen
to permeate over 10 times as fast as methane, whereas in a
polysulfone membrane, methane may permeate slightly faster
than nitrogen. The literature is replete with information
reg~rding the permeation properties of polymers. For
instance, see Hwang, et al., "Gas Transfer Coefficients
in Membranes", ~eparation Science, Volume 9, pages 4~1 to
47~ (1974) and Brandrup, et al., PolYmer Handbook, Second
Edition, Section III (1975), ~he suitability of a polymer
for, e.g., a gas separation can readily be determined using,
for instance, con~entional techniques for determining
permeabillty constants, permeabilities and separation
factors such as disclosed by Hwang, et al., in Techniques
of ChemistrY, Volume VII, Membranes in Separations (1975)
at Chapter 12, pages 296 to 322.
Typical polymers which may be suitable for making
integral, anisotropic membranes include su~stituted and
unsu~stituted polymers selected from polysulfones; poly
Cstyrenes), including styrene~containing copolymers such
as acrylonitrile-styrene copolymers and styrene-vinylbenzyl-
halide copolymers; polycarbonates; cellulosic polymers,
such as cellulose acetate-butyrate, cellulose propionate,
ethyl cellulose, methyl cellulose, nitrocellulose, etc.;
polyamides and polyimides, including aryl polyamides and
aryl polyimides; polyethers; polyacetal; polyCarylene
oxides~ such as polyCphenylene oxide~ and polyCxylylene
oxide); polyCesteramide-diisocvanate~; polyurethanes;
4~ 510
-29- 07-0433
polyesters Cincluding polyarylates), such as poly
Cethylene terephthala~e), poly(alkyl methacrylates),
polyCalkyl acrylates), poly(phenylene terephthalate),
etc.; polysulfides; polymers from monomers having alpha-
olefinic unsaturation other than mentioned above such as
polyvinyls, e.g., poly(vinyl chloride), poly(vinyl
fluoride), polyCvinylidene chloride), poly(vinylidene
fluoride), poly(vinyl alcohol), polyCvinyl esters) such
as poly(vinyl acetate) and polyCvinyl propionate), poly
Cvinyl pyridines), polyCvinyl pyrrolidones~, poly(vinyl
ethers~, polyCvinyl ketones), poly(vinyl aldehydes), such
as poly(vinyl formal) and poly(vinyl butyral), poly(vinyl
amines), poly(vinyl phosphates), and poly(vinyl sulfates);
polyallyls; poly(benzobenzimidazole); polyhydrazides;
polyoxadiazoles; polytriazoles; poly(benzimidazole);
polycarbodiimides; polypkosphazines; etc., and interpolymers
including block interpolymers containing repeating units
from the above such as terpolymers of acrylonitrile-vinyl
bromide-sodium salt of para-sulfophenylmethallyl ethers;
and grafts and blends containing any of the foregoing.
Typical substituents providing substituted polymers include
halogens such as fluorineJ chlorine and ~romine; hydroxyl
groups; lower alkyl groups; lower alkoxy groups; monocyclic
aryl; lower acyl groups and the like.
One class of polymers which may be attractive for
many gas separations are copolymers of styrene and
acrylonitrile or terpolymers containing styrene and
acrylo~litrile. Frequently, the styrene is up to about 60
or 70, say, about 10 to 50, mole percent of the total
monomer in the polymer. Advantageously, the acrylonitrile
monomer comprises at least about 20, e.g., a~out 20 to 90,
often about 30 to 80 mole percent of the polymer, Other
monomers which may be employed with styrene and
acrylonitrile to provide, e,g,, terpolymers, lnclude
olefinlc monomers such as butene, butadiene, methyl-
acrylate, methylmethacrylate, maleic anhydride, and the
like, Other mon~mers which may be employed include vinyl
chloride. The copolymers or terpolymers of styrene and
510
-30- 07-0433
acrylonitrile often have a weig~t average molecular weight
of at least about 25,000 or 50,000, say) a~out 75,000 to
500,000 or more.
One of the preferred polymers for anisotropic
mem~ranes is polysulfone due to its strength, chemical
resistance and relatively good gas separation capabilities.
Typical polysulfones are c~aracterized by having a polymeric
back~one comprised of the repeating structural unit:
o
----Rl~ 2_ r
where Rl and R2 can be the same or different and are
aliphatic or aromatic ~ydrocarbyl-containing moieties,
say, of 1 to about 40 carbon atoms, wherein the sulfur
in the sulfonyl group is bonded to aliphatic or aromatic
carbon atoms, and the polysulfone has an 'average molecular
weight suitable for film or fi~er formation, often at
least a~out 8000 or 10,000, W~en the polysulfone is not
cro~-linked, t~e molecular weight of the polysulfone is
generally less than about 500,000 and is frequently less
than about 100,000. The repeating units may be bonded,
i.e., Rl and R2 may be bonded, by carbon-to-carbon bonds
or through various link~ng groups such as
O O O O
--O~ C--,--C-N~ N--C--IN--, -O-C--, etc.
H H H
Particularly advantageous polysulfones are those in which
at least one or Rl and R2 comprises an aromatic hydrocarbyl-
containing moiety and the sulfonyl moiety is bonded to at
least one aromatic carbon atom. Common aromatic hydrocarbyl-
containing moieties comprise phenylene and substituted
phenylene moieties, bisphenyl and substituted bisphenyl
moieties, bisphenyl methane and substituted bisphenyl
methane moieties having the nucleus
-
41 51~
-31- 07-0433
~?
substituted and unsubstituted bisphenyl ethers of formula
R3 R7 R9 R5
~X~ ,.
R~\R8 R~R6
wherein X is oxygen or sulfur; and the like. In the
depicted bisphenyl methane and bisphenyl ether moieties,
R3 to R12 represent substituents which may be the same or
different and have the structure
~CI~ Z
wherein Xl and x2 are the same or different and are
hydrogen or halogen (e.g,, fluorine, chlorine, and
bromine); p is 0 or an integer, e,g., of 1 to about 6;
and Z is hydrogen, h logen Ce.g., fluorine, chlorine and
bromine), -~Y ~ R13 (in which q is 0 or 1, Y is -O-, -S-,
Q
-SS-, -0~-, or - ~-, and R13 is.hydrogen, substituted or
unsubstituted or unsubstituted alkyl, say, of 1 to about 8
carbon at~ms, or substituted or unsu~stituted aryl, say,
monocyclic or bicyclic of about 6 to 15 carbon atoms),
heterocyclic with the heteroatom being a~ least one of
nitrogen, oxygen and sulfur and being monocyclic or
bicyclic with about 5 to 15 ring atoms, sul~ato and sulfono,
especially lower al~yl-containing or monocyclic or bicyclic
aryl-containing sulfato or sulfono) phosphorous-containing
moieties such as phosphino and phosphato and phosphono,
-32- 07-0433
~14~510
especially lower alkyl-containing or monocyclic or bicyclic
aryl-containing phosphato or phospnono, amine including
primary, secondary, tert~ary and quaternary 2mines often
containing lower alkyl or monocyclic or bicyclic aryl
moieties, isothioureyl, thioureyl, guanidyl, trialkylsilyl,
trialkylstannyl, trialkylplum~yl, dialkylstibinyl, etc.
Frequently, the substituents on the phenylene groups of
the bi~phenyl met~ane and ~isphenyl ether moieties are
provided as at least one of R3 to R6, and R7 to R10 are
hydro~en. The polysulfones having aromatic hydrocarbyl-
containing moleties in general possess good the-rmal
stability, are resistant to chemical attack, and have an
excellent combination of toughness and flexibility.
Useful polysulfones are sold under trade m~ks such
~ "P-1700", and "P-3500" by Union Carbide, ~oth commercial
products are bisphenol methane~derived polysulfones
(specifically, bisphenol A-derived) having a linear chain
of the general formula
~ '~3 ~ ~ ll 3
CH3 ~
where n, representing the degree of polymerization, is
about 50 to 80. These commercially-available, bisphenol
A-derived polysulfones sometimes contain a crystallized
fraction which is believed to be an oligomer. With solvents
for polysulfone such as dimethyl formzmide and dimethyl
acet~mide, this crystallized fraction may be undissolved
and may even increase in crystal size and fraction. This
crystallized fraction may increase the difficulty in
obtaining desirable anisotropic hollow fi~er membranes.
The N-acyl~ted heterocyclic solvents used in the method of
this inver.tion appear to provide solutions containing these
polysulfone polymers which have a very small, if any,
fraction of these crystals, ~oreover, the polymer
solutions exhi~it good sta~ility a~ainst undue crystal
for~ation; hence, the polymer solution can even ~e stored
4~ 510
-33- 07-0433
prior to use. This stability provides adequate polymer
solution life to effect adequate degassification.
With respect to polysulfone polymer, e.g., P-3500,
Table A provides a list of solvents and diluents which
may be attractive for making membranes, especially hollow
fiber membranes, in accordance with the methods of this
invention.
~ABLE A
General Liquid Carrier Preferred Liquid Carrier
Compositlons, parts by Compositions, parts by
wei~ht wei~ht
l-Formyl- l-Fon~yl-
Dllue~t piperiti~e Dilue~tpiperidine Diluent
None 100 0 100 0
Formamlte 85-98 2-15 87-95 5-13
~thylene Glycol 65-98 2-35 75-95 5-25
Water 90-98 2-10 95-98 2-5
The methods of this invention are particularly
at~antageous for preparing hollow fiber membranes. With
respect to this aspect of the invention, the polymer
solution is extrudet through a spinnerette in order to
form a hollow fiber precursor, While any suitable hollow
fiber spinnerette may find application in producing the
lS hollow fi~ers of this invention, it is preferred that the
spinnerette be of the tube-in-orifice type. Tube-in-
orifice type spinnerettes are characterized ~y having a
continuous an~ular ring surrounding an interior injection
tube. Most desira~ly, the injection tu~e is concentrically
positioned within the orifice such that substantially the
same amount of polymer solution is extruded at all points
of the annular ring and the resulting hollow fi~er precursor
has an essentially uniform wall thickness. In most tube-
in-ori~ice type spinnerettes the polymer solution enters
a cavity ~ehind the annular ring to assist in the
distri~ution of polymer solution around the annular ring.
114~ 510
-34- 07-0433
S~metimes, at least about 4, preferably at least about 5,
polymer solution ports are provided, and they may desirably
be equidistantly spaced.
In spinning some polymer solutions it may be desired
to maintain the polymer solution at a~ elevatet temperature
in order to provide desirable spinning conditions. In such
instances it is desirable to provide a heating means for
maintaining the spinnerette at the desired extrusion
tempeature. Suita~le heating-means include electrical or
fluid heating jac~ets, em~edded electrical heating coils,
and the like. Since the residence time of the polymer
solution in the spinnerette is often relatively brief,
frequently the polymer solution is heated to at or near
the tesired extrusion temperature prior to entering the
spinnerette. Often the temperature of the polymer solution
in the spinnerette is at least a~out 20C but preferably
is not at such high temperatures that during t~e residence
time of the pol~mer solution at such temperatures, the
polymer solution is unduly adver~ely affected. Frequently,
the polymer solution in the spinnerette is at a temperature
in the range of about 20 to 90C, say, a~out 25 to 80C,
The size of the spinnerette will vary with the desired
inside ant outside diameters of the sought hollow i~er
mem~rane, One class of spinnerettes ma~ have orifice
diameters Coutside diameters) of a~out 300 to 1000 microns
and an inside orifice diameter Coften the injection tube
o~tside diameter) of a~out 100 to 500 microns with an
injection capillary within the injection tu~e. The
diameter of the injection capillary may vary within the
limits established by the injection tube, The projection
of the in~ection tube usually to the plane of the
spinnerette orifice. Most often, spinnerettes are
deseri~ed in terms of a configuration of the entrance to
the annular ring and the ratio of length to width of the
annular ring portion of the spinnerette, The entrance
configuration to the annular ring portion of the
spinnerette ma~ ~e a~rupt, e,g., at an angle great~er than
a~out 80 to the direction of rlow of polymer solution in
114~S~
-35- 07-0433
the annular ring portion of the spinnerette, or may be
sloped into the annular portion of the spinnerette, e.g.,
at an angle below about 80C, say, about 75 to 30, from
the direction of polymer solution flow through the annular
S spinnerette. The ratio of the length to which the annular
ring portion extends from the face of the spinnerette to
the width of the annular ring at spinnerette face may vary
within wide ranges; however, the length of the annular ring
portion of the spinnerette should not be so excessive t~at
undue pressure drop is incurred while passing the polymer
solution through the spinnerette. Frequently, these
ratios vary between about 0.7 to 2,5, say about 1 to 2.
The dimensions of the hollow fiber precursor will
depend on, for instance, the dimensions of the spinnerette,
the rate of flow o the polymer solution through the
spinnerette, the jet stretch on the hollow fi~er precursor,
and the rate and pressure of the fluid being injected into
the bore of the hollow fiber precursor. The rate of flow
of the polymer solution through the spinnerette may vary
widely tepending upon the take~up speed of the hollow
~iber. The rate of flow, however, should not be so great
as to cause fracturing of the hollow fiber precursor.
Frequently, since the size of the hollow fiber may vary
witely, e.g., from about 100 or 150 microns to loO0 or
more micrans in outside diameter and the length of hollow
~iber produced per unit time Cspinning speed or ta~e-up
speed) may also vary widely, e.g,, from about 5 to 100
meters per minute ~although higher spinning speeds can be
employed providing the fiber is not unduly stretched and
sufficient residence time is provided in subsequent
processing steps~, the spinning operations is often
described in terms of jet stretch. Spinning Cjet) stretch
is herein defined as the ratio of Ci~ the cubic centimeters
per minute of polymer solution extruded through the
spinnerette divided by the cross-sectional area of the
annular extrusion zone in square centimeters to (ii) the
length in centimeters of said hollow fiber precursor
extruded per minute ci.e., take-up speed). The spinning
4~ 51~
-36~ 07-0433
stretch is often about 0.6 to 2, say about 0.7 to 1.8,
preferably about 0.75 to 1,5.
An injection fluid is introduced into the bore of a
hollow fiber precursor in an amount sufficient to maintain
the bore of the hollow fiber precursor open. The injection
fluid may be gaseous, 0, preferably, liquid at the
conditions of the spinning of the hollow fiber, The
injection fluid should be miscible (and is prefera~ly
miscible in all proportions) with the liquid coagulant
and.with the liquid carrier. Often the injection fluid
i~ substantially a non-solvent for the polymer. In many
instances the injection fluid need not be a strong non-
sol~ent for the polymer, The injection fluid may contain
one.or more components, and frequently comprises at least
one component of the liquid coagulant in order to enhance
miscibility of the injection fluid with the liquid coagulant.
Thus, frequently, the injection fluid comprises water.
Often, the injection fluid also contains a solvent or weak
solvent for the polymer, The temperature of the injection
fluid generally approxim~tes the temperature of the polymer
solution being extruded and the temperature of the
spinnerette because of heat transfer through the wall of
the injection capillary positioned within the spinnerette.
However, with suitable insulation or other provisions to
minimize heat transfer with the injection fluid in the
spinnerette, or the use of an in;ection fluid which is
- chilled prior to pa~sing to the spinnerette, the injection
fluid may be significantly cooler than the temperature of
the polymer solution being extruded. The rate and pressure
of the injection fluid being introduced into the bore of
the hollow fiber precursor should be insufficient to result
in an~ deleterious effects to the separation capability
or structure of the hollow fiber membrane, Frequently,
the rate and pressure of injection fluid is such that the
ratio of the inside diameter of the hollow fiber to the
inside orifice diameter of the spinnerette is less than
about 2.5, and preferably this ratio is about 0,5 to 1.5
or 2.
~a o
-37- 07-0433
The spinnerette may be positioned below the level
of the liquid coagulant Cwet jet) or, prefera~ly, above
the liquid coagulant Cdry jet). When the dry jet
spinning techni~ue is employed, it is preferred that the
components of the liquid carrier be substantially non-
volatile. The distance a~ove the liquid coagulant at which
the spinnerette is positioned Cgap) may vary considerably
without noticeable effect on the properties of the hollow
fiber membrane. Frequently, t~e spinnerette is positioned
within about 20 or 30 centimeters. Desirable hollow fibers
may be produced when the face of the spinnerette is within
about 0.5 or less centlmeters of the liquid coagulant.
For sake of convenience, the spinnerette is often
positioned within about 0,5 to 15 centimeters of the
liquid coaO~ulant. The dry jet spinning technique is also
preferred as a matter of convenience when the polymer
~olution must be extruded at elevated temperatures and
the coagulation liquid is maintained at a temperature
substantially below, e,g., at least 25C, say, at least
30C, below the temperature of the polymer solution at
extrusion.
The methods of this invention are particularly
advantageous for producing anisotropic hollow fiber
membranes of polysulfone, especially polysulfones
containing ~isphenyl methane (including substituted
bi4phenyl methane) and bisphenylether Cincluding
substituted ~isphenylether) moieties. The polysulfone
hollow fiber membranes of this invention exhibit several
significant structural characteristics which enable
desirable ~trength properties to ~e combined with
advantageous fluid separation properties, The polysulfone
membranes of this invention can ~e particularly attractive
for effecting gas separations.
A dry, integral anisotropic hollow fiber mem~rane
of polysulfone in accordance with this invention has a
wall having a thin exterior skin on an open, cellular
support. The exterior skin has few, if any, pores Ci.e.,
continuous channels for fluid flow through the exterior
4~ 510
-38- 07-0433
sk~n). The cells in the cellular support of the hollow
fiber wall are preferably relatively small in all
dimensions. Desirably, the cellular support has a
substantial absence of macrovoids ha~ing a maximum length
to maxi~um width ratio greater than about 10, preferably
greater than abaut 5. The preferred polysulfone hollow
fiber mem~ranes of this invention have a substantial
absence of macrovoids,
A hollow fiber membrane for gas separations is often
evaluated in terms of its gas separation characteristics,
i.e., its selectivity for permeating one gas as compared
to at least one other gas of t~e gaseous mi~ture and flux
of the selectively permeated gas through the membrane wall,
and also its strength. These properties depend on the
chemical and physical nature of the material of the hollow
fiber as well as the structure of t~e hollow fiber. Since
morphological structures which play important roles in
determining the gas separation characteristics cf the
hollow fiber membrane may be on the order of tens of
angstroms or less in dimension, such structures can not
~e v~sually perceived even using the best optical
microscopic techniques available. Such small structures
are thus termed "submicroscopic" structures.
The combination of microscopic techniqueq Cparticularly
scanning electron microscopy and transmission electron
microscopy~ and observable performances in characteriæing
hollow fiber membrane structures on a gross basis and a
submicroscopic basis can be highly useful. A particularly
useful microscopic technique for characterizing hollow fiber
membrane structures is scanning electron microscopy. To
assist in understanding the use of scanning electron
microscopy in describing hollow fiber membranes in
accordance with this invention, references can be made to
the provided ~igures, Figurec 1 and 2 are scanning electron
microscopic photographs of hollow fiber membranes in which:
Figures la to lc depict a hollow fiber m~mbrane
prepared from a polymer solution containing 32 weight
percent polysulfone ~^3500~, a~out 7 weight percent
4~ 51~
-39- 07-0433
form2mide and about 61 weight percent l-formylpiperidine.
Figure la shows the cross-section of the hollow fiber at
a magnification of about 300 times. Figure lb shows a
segment of the cross-section of the hollow fiber at the
exterior edge of the hollow fiber and is at a magnification
of about 20,000 times. Figure lc shows the structure of
the cross-section of the hollow fiber in an area
approximately in the middle of the wall of the fiber and
is at a magnification of about 20,000 times,
Figure 2 depicts a cross-section of a hollow fiber
mcmbrane prepared from a polymer solution containing 36
weight percent polysulfone ~P-3500~ and 64 weight percent
l-formylpiperldine and is at a magnification of about 300
t~mes
Samples of hollow fiber membranes Csuch as depicted in
the figures2 for examination by scanning electron microscopy
can conveniently be prepared by thoroughly drying the hollow
fiber, immersing the hollow fiber in hexane and immediately
placing the hollow fiber in liquid nitrogen such that the
hollow fiber can be fractured. Upon obtaining a clean
fracture of the hollow fi~er, the specimen can be mounted
and then sputter coated with a gold and palladium mixture
Cusing, for instance, a Hummer II sputter coater). The
coating procedure usually results in a coating of about S0
to 75 angstroms being placed on the hollow fiber sample.
Accordingly, the dimensions of very s~all features may be
obscured ~y the coating,
As can be seen especially from Figure 1, the structure
of the hollow fiber membranes is characterized by having
a relatively dense structure adjacent the exterior surface
with an underlying open cellular support. An increase in
the size of the cells in the hollow fiber wall may be seen
as the distance from the exterior surface increases to a
generally maximum size in a middle region within the
hollow fi~er wall, The predominant structure of the cells
in the middle region of the cross-section of the hollow
fiber wall may ~e observed to ~e intercommunicating cells
Ccells with open passage for gas flow into adjacent cells),
f~ f ~
-40- 07-0433
SiO
i.e., an open, cellular structure. This intercou~unica~ion
between cells, however, m2y not be capable of being
visually perceived if the cells are very small, e.g.,
less than 0.1 micron in ma~or dimension. This open,
cellular structure ena~les fluids to readily pass through
the hollow iber wall wit~ minimal r~sistance.
Advantageously, the exterior skin (and interior skin, if
any) provid~s the major portion of resistance to fluid flow
through the membrane wall.
The d~mensions of the cells, especially larger cells
which often occur in the middle region of the hollow fiber
wall, can frequently be estimated using, e.g., scann~ng
electron microscopic techniques, In estimating the
approximate size and configuration of the cells the
observable passages between individual cells are consïdered
to be defects in the cell wall, and therefore, the cell
dimensions do not continue through these passages. Another
feature of the cells which may have impor~ance in evaluatirg
the strength of the mem~rane is the thickness of the 20 material of the cell wall defining the larger cells in the
hollow fiber membrane wall, These estimations can be
conducted by visually inspecting photographs of the cell
~tructure or by using available computer scanning techniques
such as image analyzers to inspect and analyze the
~5 photograph. Suitable image anal~Jzers include image
analyzers useful in analysis of conventional textile fibers
such as the Quantimet~720-2 image analyzer available from
Quantimet, Munsey, New York.
The configuration of the cells, especially in the
larger cells which often exis~ in the middle region of
the hollow fiber wall, can vary widely. A convenient
manner for characterizing the configuration of the ceLl
i9 by dividing the cross-sectional cell area by the square
of perimeter of the cell at that cross-section, to provide
a configuration ratio. Thus the configuration ratio of a
perfect circle would be about 0.8, a square would be
about 0.06, and a rod, 0.01 or less. When the
- configuration ratio is in the range of 0.03 to 0.04,
*--Trade~ark
114~510
-41- 07-0433
the cross-section o the cells are often roughly polygonal
in appearance. Such analyses can be conducted by visually
inspecting photographs of the cell structure of the hollow
fiber wall or by the use of an image analyzer to inspect
photographs. Conveniently, the configuration ratio can be
determined by analyzing a random segment o the cross-
section of the hollow fiber wall, which segment contains
cells which are sufficiently large to ~e inspected, and
which segment has an absence of macrovoids. The area of
a segment examined in order to be representative is
generally at least about 25 square microns (e,g., a segment
ha~ing a minimum dimension of at least about 5 microns).
In many desirable hollow fiber membranes in accordance
with this invention the mean canfiguration ratio is at
least about 0.025, and in many instances is at least about
0.03 or even at least about 0.035. Preferably, segments
of the cross-section of the hollow fibers which contain
the largest cells Cexcluding macrovoids) will have a mean
configuration ratio of at least about 0.03, e,g,, at least
about 0.035.
When the cell sizes in a segment of a cross-section
of the hollow iber wall are greater than abaut 0.1, say,
greater than 0.2, micron, analysis of photographs of cell
structures as, e.g,, perceived through scanning electron
microscopy, may be also useul in determining the ratio of
mean wall thic~ness to mean cell cross-sectional area.
In general, the greater the ratio of thickness of the
polymer supporting the cell to t~e cross-sectional area
of the cell, the stronger the ~ollow fiber. Frequently,
this ratio, especially for the largest cells in the hollow
fiber membrane cross-section, will be at least about 0.05,
say, at least about 1, and sometimes in the range of about
2 to 20, reciprocal centime~ers. In analyzing photographs
of smaller cells and smaller wall thicknesses of scanning
electron microscope images, the thickness of the
reflecting coating may become significant and should be
taken into account. Often, at least a majoritv of the
cells capable of being observed in suc~ photogra?hs ~ill
4~ 5~0
-42- 07-0433
also have observable passages communicating with adjacent
cells,
Preferably a volume ~as determined by cross-sectional
area) majority of wall of the hollow fiber consists of cells
having a mean major dimension less than about 2 ~icrons.
Frequently, at least about 75, say, at least about 9~,
volume percent of the wall consists of cells having a
major d~mension less than a~out 2 microns, prefera~ly less
than 1.5, say, less than 1, micron, In some advantageous
hollow fiber membranes, a substantial absence of cells
having major dimensions greater than 2 or 1,5 microns
exists, As stated earlier, the size ran8e o~ cells in the
wall of the hoIlow fiber membranes usually varies
substantially from a size too small to be resolved through
scanning electron microscopy to cells having major
dimensions of 2 microns or greater. In s~me hollow fibers
in accordance with this invention, the predominant major
dimensions of the larger cells in the hollow fiber membrane
wall are less than about 0,75 microns, and in some
instances essentially all of the cells in the zone of the
largest cells in t~e hollow fiber membrane wall have major
d~mensions o about 0,1 to 0,7 microns. O~ten, this zone
o~ the largest cells is at least about 30, say, at least
about 40, e,g., up to about 50 or 75, volume percent of
the wall of the hollow fiber, Often, desirable hollow
iber mem6ranes have mean major dimensions of cells which
are relatively uniform regardless of the segment Csay,
o~ at least about 25 square microns in area with a minimum
dimension of at least about 5 microns) observed in at
least a volume majority (say, at least about 75 volume
percent) of the hollow fiber wall.
In some instances hollow fiber membranes can exhibit
adequate resistance to collapse even though macrovoids may
be present. Macrovoids, when they appear in the hollow
fiber, can vary widely in shape and position within the
hollow fiber wall.. Macrovoids which more closely approach
the shape of a circle can generally be tolerated to a
greater e~tent without an undue reduction in resistance
114~S10
-43- 07-0433
to collapse than long, finger-like, macrovoids. Desirably,
if macrovoids occur, they occur substantially only in
the thickness of the hollow fiber membrane from the
interior surface to about 0.5 or 0.75 the distance of
the exterior wall. Preferably, the hollow fiber membrane
has a substantial absence of macrovoids ha~ing a maximum
length to maximum width ratio greater than about 10, say,
about 5, The major dimension of the macrovoids is
preferably less than about 0.4, say, less than 0.3, times
the thickness of the hollow fiber wall.
Scanning electron microscopy may sometimes be useful
in examining the porosity of the internal (bore side) skin
of the hollow fiber membrane. Preferably, the internal
s~in is highly porous to enable permeating gases to pass
into the bore of the hollow fiber with little resistance.
A useful microscopic technique for estimating extremely
small structural features, e.g., the porosity of the
external surface Cand possibly the internal surface) of
the hollow fiber membranes is the surface replicate
technique, In this technique, a metal coating, e.g., a
plat~num coating, i9 applied to the surface, and then the
pol~meric hollow fiber is dissolved from the metal coating.
The metal coatlng is then analyæed using transmission
electron microscopy, e.g., at magnifications of 50,000
times or greater, in order to determine the surface
characteristics of the hollow fi~er whic~ have been
replicated by the metal coating, In general, the most
preferred hollow fiber membranes of this invention provide
smooth surface replicates of the external surface having a
substantial absence of an~malies, or irregularities, (which
could represent pores~ greater than about 100, often
greater than about 75 or even 50, angstroms.
A particularly valuable analytical tool for kinetic
evaluations of hollow fiber membranes is the permeability
35 o~ gases through the membrane. The permeability of a
given gas through a membrane of a given thickness C~) is
the .volume of gas, standard temperature and pressure CSTP),
which passes through the membrane per unit area of mem~rane
f`~ r
-44- 07-0433
11415:10
per unit of time per unit of partial pressure differential.
One method for reporting permeabilities is cubic
centlmeters (STP) per sqùare centimete~ of membrane area
per second for a partial pressure diffexential of 1
centimeter of mercury across the membrane (cm3/cm2-sec-
c~g) Unle~s o~herwise noted, all permeabilities are
reported herein at standard temperatures and pressures
and are measured using pure gases. The permeabilities are
reported in gas permea~lon units ~GPU) which is cm3(STP)/
cm2-sec-~g x 106; thus, 1 GPU is lxlO~6cc(STP)/cm2-sec-
cmHg. Several of the many techni~ues available for
determining permeabilities and permeability constants
Ce.g., the volume of gas CSTP) passing through a given
thickness of the material of the membrane per unit area
per unit time per unit pressure differential across the
thickness) are disclosed by Hwang, et al., Techniques of
~hemistry, Volume VII, Membranes in Separations, John
Wiley & Sons, 1975 at Chapter 12, pages 296 to 322. The
permeability of a given gas reflects permeation of that
gas through the membrane xegardless of the mechanism by
which the ~as passes through the membrane.
One useful kinetic relationship which is indicative
of the freedom from pores in ~he separating, or barrier
layer, and the range of sizes of the pores in the barrier
layer, is by determining t~e ability of the mem~rane to
separate different molec~lar weight gases, e.g,, low
molecular weight gases, of su~stantially dif~erent molecular
weights, Suitable gas pairs for this analysis often
in~lude one of molecular hydrogen and helium and one of
molecular nitrogen, carbon monoxide or carbon dioxide
Prefera~ly, the hig~er molecular weight gas of the gas pair
selected will ~ave a permeability constant ~i,e., the
intrinsic permeability constant) of the polysulfone at
least about 10 times less than the intrinsic permeability
constant of the lower molecular weight gas in the
polysulfone. The analysis can be conveniently conducted
- at ~mbient temperatures Ce.g , room temperature of about
25C~ at a pressure of about 7.8 atmospheres absolute on
'
. . -~
~14~5iO
-45- 07-0433
the exterior side of the hollow fi~er membrane and about
1 atmosphere absolute on the bore side of the hollow fiber
membrane. The polysulfone hollow fiber membranes of this
invention exhibit for at least one pair of gases a
permeability ratio of ci) the permeability of the lower
molecular weight gas (P/Q)L~ divided by the permeability
of the higher molecular weight gas CPIQ)HJ to (ii) the
square root of the molecular weight of the lower molecular
weight gas, ~ divided ~y the square root of the
molecular weight of the higher molecular weight gas, ~ ,
of at least about 6, frequently at least about 7.5. The
theoretical maxi~um permea~ility ratio is the ratio of
the intrinsic separation factor for the gas pair to the
quotient of the square root of the molecular weight of the
lower molecular weight gas divided ~y the square root of
the molecular weight of t~e higher molecular weight gas.
An intrinsic separation factor as referred to herein
is the separation factor for a material which has no
channels for gas flow across the material, and is the
highest achievable separation factor for the material.
Such a material may be referred to as being continuous or
non-porous The intrinsic separation factor of a material
can be approximated by measuring the separation factor of
a compact membrane of the material, However, several
difficulties may exist in the de~ermination of an intrinsic
separation factor including imperfections introduced in
the preparation of the compact me~brane such as the presence
of pores, the presence of fine particles in the compact
membrane, undefined molecular order due to variations in
membrane preparation, and the like. Consequently, the
determined intrinsic separation factor can be different
than the intrinsic separation factor. Accordingly, a
"determined intrinsic separation factor" as employed herein
refers to the separation factor of a dry com~act membrane
of the material.
Another useful method to characterize hollow fiber
membranes on a kinetic basis is ~y determining the
permeabilities of a pair of low molecular weight gases
1~41510
-46- 0?-0433
having approximately the same molecular weights but one
has a permea~ility constant in the polysulfone at least
about 5, preferably, at least about 10, times greater
than that of the other gas. Typical gas pairs which ~ave
approximately the same molecular weight ~ut substantially
tiffer in permeability constants for many polysulfones
suitable for hollow fiber memBsanes include ammonia and
methane; car~on dioxide and propane; etc, This
relationship can be expressed as the quotient of the
difference between the permeability of the more readily
permeated gas, CP/Q)F, and the permeability of the less
readily permeated gasl CP/Q~S, divided by (P/Q)S times
the permeability constant of the less readily permeated
gas, PS, divided by the permeability constant of the more
readily passed gas PF:
r(P/Q)F - CP/'~)S1 ~PS 1
L 'P'~'S J LPFJ
In many instance~, this relationship is at least about
0.001, say, at least about 0.01, preferably, at least
about 0.02, This relationship is indicative of the
material of the hollow ftber membrane effect1~g at least
a portion of the separation,
~ nother useful kinetic analysis of a hollow fiber
mem~rane is the ratio of the permeability of a gas to
the permeability constant of the polysulfone for the gas:
(P/ ~ )
p
The gas employed to determine this relationship preferably
readily permeated the membrane, e,g,, hydrogen, helium,
ammonia, or. the like. Often this ratio is at least about
5x104, say, at least about lx105, preferably at least
about 2x105, up to about lx106, say, about lx105 to 0.6x105,
reciprocal centimeters. This relationship is indicative of
a low resistance to gas flow through the hollow fiber
membrane structure and thus the high permea~ilities of
1141S10
-47- 07 0433
the desired permeating gas that can be achie~ed.
The structure of the hollow fiber membranes of this
invention can also be-o~served through other kinetic
e~aluations. For instance, dried hollow fiber membranes
exhibit ~ery little li~uid water permeability tue to the
existence of few, if any, pores in the separating or
barrier, layer. The water permeability is often less
than about O.5xlO-6, or even less than about 0.2x10-6,
say, less than about 0,lx10_6, and sometimes less than
0,Olx10-6 cm3 Cliquid~/cm2-sec-cmHg, even after soaking
in room temperature C25C) water for 4 days, The water
permeability determination can conveniently be at room
temperature with a pressure drop of 3,4 atmospheres from
the feed side to permeate side of the membrane at an
absolute pressure of about 4.4 atmospheres on t~e feed
side. Frequently, the maximum pore size is less than
about 250 angstroms, and often is less than about 150,
e.g,, less than about 100 angstroms.
A method for evaluating the size and ope~ness of the
cellular support i9 by sub~ecting the hollow fiber membrane
to the conventional Brunauer, Emmett and Teller CBET)
adsorption analysis for determining surface areas. In a
BET analysis, a substantially monolayer of a gas, e.g.,
nitrogen, is sought to be adsorbed on the available
internal surface of the hollow fiber and the amount of
adsorbed gas is determined and is believed to be
proportional to the available surface area. In the BET
analysis technique described herein, the analysis is
substantially conducted such that the internal surface
area of cells which are not open does not contribute
significantly to determination of the available surface
area, Smaller cells provide more surface area per gram
of polymer than larger cells, Consequently, a BET
analysis can provide an indication of the openness of
cells which are too sm~ll to be resolved by, e.g.,
scanning electron microscopy. Many hollow fiber
membranes in accordance with this invention eghibit
surface areas as determined ~ the BET method of at least
1141510
-48- 07~0433
about 18 or 20, preferably at least about 22, square meters
per gram. At very high BET determined surface areas, e.g,,
above 50 or 7~, square meters per gram, the hollow fiber
is often observed to have a fibrillar structure instead
of a cellular structure. Fibrillar structures can sometimes
be relativély weak in comparison to ceIlular structures.
Commonly, the hollow fiber membrane internal surface area
as determined by a BET analysis is in the range of about
18 or 22 to 30 square meters per gram. Hollow fibers which
have a predominantly closed-cell structure often exhibit
BET determined surface areas of less than about 18 or 20
square meters per gram, - Another useful method for
e~aluating surface areas determined by BET analysis is in
terms of surface area per unit volume of hollow fiber wall,
Frequentlyj desirable hollow fiber membranes of the
invention exhibit surface areas as determined by a BET
analysis of about 10 to 30, say, about 10 to 20, square
meters per cubic centimeter of hollow fiber wall volume.
The structures of the polysulfone hollow fiber
membranes of this invention enable the hollow fibers to
withstand large pressure differentials from the exterior
to the ~ore of the hollow fiber membrane, i,e,, the hollow
fiber membranes exhibit high collapse pressures. Thus,
the hollow fiber m~mbrane can have a relatively low ratio
of thickness Cwall) to outside diameter while maintaining
a desired structural strength and thereby provide an
advantageously large bore diameter to minimize the pressure
trop to gases passing within the bore, The collapse
strength fr~m external pressure of a tube having a foamed
wall structure depends upon the strength of the material
of the tu~e, the wall thickness of the tube, the nature
of the foamed wall structurè, the outside diame~er of the
tube, the density of the foam and the uniformity of the
hollow fiber configuration ci,e,. whether the outside and
lnside configurations are concentric and circular~, In
general, the collapse pressure of the hollow fiber
membranes of this in~ention exhibits an approximation
relationship to the strengt~ of the material of the
f ~
-49- 07-0433
1~41510
hollow iber and the wall thickness divided by the outside
diameter of the hollow fiber Ct/D). This approximation
relationship can be expressed in terms of the tensile
strength o~ the material of the hollow fi~er and the cube
of the ratio of the wall thickness to the outside diameter.
Similar relationships can exist with respect to other
strength proper~ies of the material of the hollow fiber
such a~ modulus of elasticity and the like. Frequently,
the holl~w fi~er mem~ranes of this invention exhibit a
collapse pressure (kilograms/square centimeters) greater
than abou~ 4 TS ~)3,where TS is the tensile strength at
yield (or at break if appropriate) in kilograms per square
centimeter More advantageous hoIlow fiber membranes in
accordance with this invention also exhibit a collapse
pressure greater than about 10C~)CTS) ~)3 wherein ~ i~ thè
volume fraction of material of the hollow fiber membrane
in the wall of the hollow fi~er membrane. The volume
fraction of the material o~ the hollow fiber membrane
(~) is therefore 1 minus the quantity of void volume
~volume percent)/100.
Ani~otropic membranes can be prepared in accordance
with the method of this invention to minimize the
separating, or barrier, layer thickness. Frequently, the
tendency to produce pores in the membranes is increased
when the barrier layer is decreased. In accordance with
the teachings of Henis, et al., U. S. Patent Number
4,230,463, granted October 28, 1980, a coating may be
provided in occluding contact with a gas separation mem-
brane containing pores to enhance the selectivity of
separation exhibited by the gas separation membrane wherein
the coating material does not significantly effect the sep-
aration. Usually, sùitable membranes for coating in accor-
dance with the invention of Henis, et al., exhibit a per-
meability ratio as defined above of at least about 6.5, fre-
quently, at least about 7.5. In many instances, the uncoatedmembranes which are most advantageously coated exhibit a
1~4~S10
-50- 07-0433
permeability ratio of less than about 40, e.g., less
than about 20 or 25 The ratio of the permea~ility of a
gas (preferably a gas which readily permeates the material
of the membrane) to the permeabil~ty constant of the
material of the membrane for the gas is at least about
lx105, preferably, at least about 2x105, up to, say,
lx106, reciprocal centimeters. For instance, advantageous
polysulfone Ce.g., P-3500) hollow fi~er me~branes (and
many other hollow ~iber mem~ranes) for coati~g exhibit a
hydrogen flux of a~out 100 to 1200 GPU or more and exhibit
ratios of hydrogen permeability to methane permea~ility
of about 1.5 or 2 to a~out 10, say, a~out 2 to 7.
In many instances the selectivity of separation for
at least one pair of gases after coating the mem~rane is
at least about 40 percent, sa~, at least a~out 50 percent,
of that exhibited by an essen~ially non-porous membrane
of the same material, i,e., a non-porous membrane through
which gases pass essentially only ~y interaction with the
material of the membrane. The selectivity of separation
o the coated membrane mæ~ ~e at least a~out 35, e.g.,
at lea~t about 50 or 100, percent greater than the
selectivity of separation for said pair of gases which
coult be provided ~y the materlal of the coating when in
the form of an essentially non-porous mem~rane. Moreover,
after coating, the flu~ which can be obtained using the
coated membrane is not unduly reduced, e.g , the ratio
of permea~ility of a gas to the permea~ility constant
of the material of the membrane is preferably at least
a~out 5x104, pre~era~ly, at leact about lx105, reciprocal
cent~meters.
Particularly advantageous materials or coatings to
enhance the selectivity of separation of the membranes
do not unduly reduce the permeation rate (flux) of the
gas or gases desired to ~e permeated through the wall of
membrane, Often, the materials for the coatings have
relatively high permea~ility constants The material of
the coating should ~e capable of providing occluding
contact with the exterior surface of the membrane. For
~14~510
-51- 07-0~33
instance, when applied it should sufficiently wet and
adhere to the hollow fi~er to ena~le occluding contact
to occur. The wetting properties of t~e material of the
coating can ~e easily determined ~y contacting the material
of the coating, either alone or in a solvent, with the
material of the membrane. Moreover, based on estimates
of the a~erage pore diameter in the separating thickness
of the mem~rane, mat'erials for t~e coating o~ appropriate
molecular size can be chosen, If the molecular size of
the material of the coating is too large to be accommodated
by the pores, the material may not be use~ul to provide
occluding contact, If, on the other hand, the molecular
size of the material for the coating is ~oo small, it
may be drawn through the pores of the membrane during
coating and/or separation operations. When the pores are
in a wide ~ariety of sizes, it may ~e desira~le to employ
a polymerizable material for the coating material which is
polymerized after application to the membrane, or to
employ two or more coating materials of different molecular
sizes, e.g,' ~y applying the materials of the coating in
order of their increasing molecular size$,
The ~aterials for the coating may ~e natural or
synthetic substances, and are often polymers, and
advantageously exhi~it the appropriate properties to
provide occluding contact with the mem~rane. Synthetic
su~stances include ~oth addition and condensation polymers.
Typical of the useful materials w~ich can comprise the
coating are polymers which can ~e suBstituted or
unsu~stituted, and which are solid or liquid under gas
separation conditions, and include synthetic ru~bers;
natural ru~bers; relatively high molecular weight and/or
high ~oiling liquids; organic prepolymers; p'olyCsiloxanes)
Csilicone polymers); polysilazanes; polyurethanes; poly
Cepichlorhydrin~; polyamines: polyimines; polyamides;
acrylonitrile~containing copolymers such as polyCa-chloro-
acrylonitrile~ c'opolymers; polyeste-s (including polylactams
and polyarylates), e.g., poly(alkyl acrylates) and poly
(alkyl methacrylates) wherein the alkyl groups have, say,
~4~510
-52- 07-0433
1 to about 8 carbons,polysebacates, polysuccinates, and
alkyd resi~s; terpinoid resins; linseed oil; cellulosic
polymers; polysulfones, especially aliphatic-containing
polysulfones; poly(alkylene glycols) such as poly(ethylene
glycol), poly(propylene glycol), etc.; poly(alkylene)
polysulfates; polypyrrolidones; polymers from monomers
hav~ng a-olefinic unsaturation such as poly(olefins),
e.g,, poly(ethylene), poly~propylene), poly(butadiene),
poly(2,3-dichlorobutadiene), poly(isoprene), poly
(chloroprene), poly~styrene) including poly(styrene)
copolymers, e.g., styrene-butadiene copolymer, polyvinyls
such as polyCvinyl alcohols), poly(vinyL aldehydes) Ce.g.,
polyC~inyl formal) and poly~vinyl butyral)~, polyCvinyl
ketones) (e.g., poly~methylvinylketone)), poly(vinyl esters)
Ce.g., poly(yinyl benzoate)), poly(vinyL halides) ~e.g.,
poly~inyl bromide)), poly(~inyl halides), poly(vinylidene
carbonate), polyCN-vinylmaleimide), etc., poly(l,5-cyclo-
octadiene), poly(methylisopropenylketone), fluorinated
ethylene copolymer; poly(arylene oxides), e.g., poly
~xylylene oxide); polycarbonates; polyphosphates, e.g,,
polyCethylenemethylphosphate); and th~ like, and any
lnterpolymers including block interpolymers containing
repeating units from the above, and grafts and blends
containing any of the foregoing, The polymers may or may
not be polymerized after application to the membrane.
Particularly useful materials for coatings comprise
poly(siloxanes). Typical poly(siloxanes) can comprise
aliphatic or aromatic moieties and often have repeating
units containing 1 to about 20 carbon atoms. The molecular
weight of the poly(siloxanes) may vary widely, but is
generally at least about 1000. O~ten, the poly(siloxanes
have a molecular weight of.about 1,000 to 300,000 or more
when applied to the membrane. C~mmon aliphatic and
aromatic poly(siloxanes) include the poly(monosubstituted
and di ubstituted siloxanes), e.g,, wherein the substituents
are lower aliphatic, for instance, lower alkyl, including
cycloalkyl, especially methyl, ethyl, and propyl, lower
alkoxy; aryl including mono or ~icyclic aryl including
~ . ,
114~S10
-53- 07-0433
bisphenylene, naphthalene, etc.; lower mono and bicyclic
aryloxy; acyl including lower aliphatic and lower aromatic
acy~; and the like, The aliphatic and ar~matic substituents
may be substituted, e.g., with halogens, e.g., fluorine,
chlorine and br~mine, hydroxyl groups, lower alkyl groups,
lower alkoxy groups, lower acyl groups, glycidyl groups,
amino groups, vinyl groups, and the like. The poly
(siloxane) may be cross-linked in the presence of a cross-
linking agent to provide a silicone rubber, and the poly
Csiloxane) may be a copolymer with a cross-linkable comonomer
such as a-methylstyrene to assist in the cross linking.
Typical catalysts to promote cross-linking include the
organic and inorganic peroxides. Cross-linking may occur
prior to application of the poly(silo~ane) to the membrane,
but preferably the poly(siloxane) is cross-linked after
being applied to the membrane. Frequently, the poly
(siloxane) has a molecular weight o~ about 1,000 to 100,000
or more prior to cross-linking. Particularly advantageous
poly(siloxanes) comprise poly(.dimethylsiloxane), poly
Chydrogenmethylsiloxane), poly~phenylmethylsiloxane),
polyCtrifluoropropylmethylsiloxane), copolymer o~ a-methyl-
styrene and dimethylsiloxane, and post-cured poly
t~methylsiloxane)-containing silicone rubber having a
molecular weight of about 1,000 to 50,000 or more prior to
cross-linking. Some poly(siloxanes) do not sufficiently
wet the material of the hollow fiber to provide as much
occluding contact as is desired. ~owever, dissolving or
dispersing the poly(siloxane) in a solvent which does not
substantially affect the material of the membrane can
facilitate obtaining occluding contact. Suitable sol~ents
include normally liquid alkanes, e.g., pentane, isopentane,
cyclohexàne, etc.; aliphatic alcohols, e.g., methanol,
some halogenated alkanes and dialkyl ethers; dialkyl ethers;
and the like; and mixtures thereof.
The coating may be in the form of an essentially
non-interrupted membrane, i.e., an essentially non-porous
membrane, in contact with the anisotropic membrane, or
the coating may ~e discontinuous, or interrupted.
1~4~S10
-54- ~7~0433
Preferably, the coating is not so thick as to adversely
affect the performance of the membrane, e,g., by causing
a~ undue decrease in flux or by causing such a
resistance to gas flow that the separation factor of
the coated membrane i9 essentially that of the coating.
Often the coating may have an average thickness of up to
about 50 microns. When the coating is interruptedt of
course, there may ~e æ eas having no coating material. The
coating may often have an average thickness ranging from
ab~ut 0.0001 to 50 microns, In s~me instances, the average
thickness of the coating is less than about 1 micron, and
may even ~e less than about 5000 angstroms. The coating
may comprise one layer or at least two separate layers
which may or may not ~e of the same materials, The coating
may be applied in any suitable manner, e.g., ~y a coating
operation such as spraying, brushing, immersion in an
essentîally liquid substance comprising the material of the
coating or the like. The material of the coating is
preferably contained in an essentially liquid substance
when applied and may ~e in a solution using a solvent for
the material of the coating which is substantially a non-
solvent for the material of the membrane. Conveniently,
the coating may be applied after the assembly of the
memBranes for later installation in a permeator, or even
after installation in the permeator, to minimize the
handling of the coated membrane.
It is frequently possi~le to treat the exterior
surface of a mem~rane with a densifying agent, e.g,, a
plasticizer, swelling agent, non~solvent, or a solvent
Cpreferably a weak solvent~ for the material of the
anisotropic membrane to densify the exterior surface of
the mem~rane. The selection of a suitable densifying
agent will depend upon the polymer of the membrane.
Densifying agents which may find application include
liquids and gases such as ammonia, hydrogen sulfide,
acetone, methyl ethyl ketone, methanol, isopentane,
cyclohe~ane, hexane, isopropanol, dilute mi~tures of
solvents for the polymer Ce.g., toluene, benzene,
114~510
-55- 07~0433
methylene chloride) in non-solvents for th~ polymer, and
the like. The treatment may ~e conducted before or after
the membrane has been dried, and may ~e conducted over a
wide range of temperatures, Temperatures of about 0 to
50C, say, about 10 to 35C, are usually ~mplo~ed.
Treatment of the mem~rane with a densifying agent prior
to coating to increase selectivity may be advantageous in
some instances.
The mem~ranes of this invention are widely applica~le
in gas separation operations, particularly the separation
of low molecular weight gases, by interaction of the
permeating gases with the m~terial of the membrane
Gaseous mixtures which may be employed in gas separation
operations ar~ comprised of gaseous substances, or
substances that are normally liquid or solid but are vapors
at the temperatures under which the separation is conducted.
Typical gas separation operations which may be desired
inclute separation~ of, for example, oxygen from nitrogen;
h~drogen from at least one of carbon monoxide, carbon
dioxide, helium, nitrogen, oxygen, argon, hydrogen sulfide,
nltrous oxide, ammonia, and hydrocar~on of 1 to about 5
carbon atoms, especially methane, ethane, and ethylene;
ammonia from at least one of hydrogen, nitrogen, argon,
and hydrocarbon of 1 to about 5 carbon atoms, e,g.,
methane; carbon dioxide from at least one of carbon
monoxide and hydrocar~on of 1 to a~out 5 carbon atoms,
e.g,, methane; helium from hydrocar~on of 1 to a~out 5
car~on atoms, e.g., methane; hydrogen sulfide from
hydrocarbon of 1 to about 5 car~on atoms, for instance,
methane, ethane, or ethylene; and carbon monoxide from at
least one of hytrogen, helium, nitrogen, and hydrocar~on
of 1 to a~out 5 carbon atoms, It i5 emphasized t~at
membranes in accordance with this invention may find
beneficial application in the separation operations using
other gas mixtures.
The following examples are provided to assist in the
understanding of the invention and are not in limitation
of the invention, All parts and percentages of gases are
4~ 510
-56- 07-0433
by volume and all parts and percentages of liquids are
by weight, unless otherwise noted. All gas permeabilities
are determined using substantially-pure gases with a shell
side pressure of about 70 atmospheres absolute and a~out 1
atmosphere at the bore side of the hollow fiber membrane
unless otherwise stated.
510
_57- 07-0433
EXAMPLE L
Thirty two parts by weight o polysulfone (P-3500,
obtainable from Union Carbide Corporation) and 68 parts
by weight of a liqu~d carrier consisting of 90 parts by
weight of l-formylpiperidine and 10 parts by weight of
formamide (reagent gr~de) are charged to a heated dope
mixer having a dry nitrogen atmosphere. Prior to being
charged to the dope mixer, the polysulfone polymer is
dried at about 125C for five to seven days under vacuum
(e.g., at an absolute pressure of less than about 10 or
20 millimeters of mercury). The formylpiperidine has a
moisture content of about 0.45 grams per 100 milliliters
and a formamide, about 0.15 grams per 100 milliliters. The
exposure of formamide to air during the charging of the
heated dope mixer is minimized to avoid degradation of the
for~mamide. In the heated dope mixer the polymer solution is
maintained at about 80C to 100C for a time sufficient to
completely dissol~e the polysulfone Cresidence times of about
8 hours are generally employed), The polymer solution is
transferred to a deaerator having a dry nitrogen atmosphere
which is heated to about 80C, The deaerator contains a
conical-shaped partition with a cup at the apex of the
partition. T~e polymer solution when fed to the holding
tank is introduced into the cup at ~elow the level of polymer
solution in the cup and the polymer solution flows over the
edge of the cup and down the conical-shaped partition in
the form o~ a thin film. The polymer solution passes from
the lower edge o the conical-shaped partition onto a wire
mesh which leads into the polymer solution in the deaerator.
A recycle stream of the polymer solution in the deaerator
is maintained during deaeration. The deaerator is maintained
at an absolute pressure of about 250 to 350 millimeters of
mercury. Satisfactory degassing may also be achieved using
substantially atmospheric pressure in the deaerator.
Adequate deaeration is generally achieved in less than about
7 hours residence time in the deaerator.
114~510
-5~3- 07-0433
The deaerated polymer solution is pumped through a
Zenith-type H pump at a rate of 8.6 grams per minute to a
tube-in-orifice-type spinnerette having an orifice diameter
of 533 microns, an injection tube outside diameter of 203
microns and an injection capillary diameter of 127 microns.
The spinnerette has five equidistant polymer solution
entrance ports positioned behind the annular extrusion
zone and is maintained at a t~mperature of approximately
50 to 58C by the use of an external, electrical heating
jacket; and the temperature of the polymer solution
approximates this temperature. Deionized water at about
ambient temperature C20 to 25C) is fed to the spinnerette
at a rate of about 1.2 millîliters per minute. While in the
spinnerette, the deionized water is warmed by heat transfer.
The spinnerette is positioned about 10.2 centimeters above
the coagulation bath. The hollow fiber precursor is extruded
at a rate of a~out 42,7 meters per minute~
The hollow fiber precursor passes downwardly from the
spinnerette into an elongated coagulation bath. The
coagulation bath contains substantially tap water and
sufficient fresh tap water is added and coagulation bath
liquid Cliquid coagulant) purged to maintain t~ concentra-
tion of l~formylpiperidine in the coagulation bath less
than about one weight percent, The liquid coagulant is
maintained at a temperature of about 2 to 4C. The hollow
fiber precursor passes vertically downward into the liquid
coagulant for a distance of about 7.8 centimeters, passes
around a roller to a slightly upwardly slanted path through
the liquid coagulant and then exits from the coagulation
bath. The distance of immersion in the coagulation bath
is about 1.4 meters.
The hollow fiber from the coagulation bath is then
washed with tap water in three sequential godet baths,
In the first godet bath, t~e hollow fiber is immersed for
a distance of about 7.3 meters. The first godet bath is
maintained at a temperature of about 2 to 4C and the
concentration of l-formylpiperidine in ~he bath is
maintained below about three weight percent by the addition
-
510
_59_ 07-0433
of fresh water and purging of the washing medium. The
second and third godet baths are at ambient temperature
(e.g., about 15 to 25C depending on the temperature
of the tap water and the laboratory room temperature) and
fresh water is continuously added to the godet baths. The
wet hollow fiber has an outside diameter of about 440
microns and an inside diameter of about 155 microns.
The hollow fiber, while being maintained wet with
water, is wound on a bobbin using a Leesona winder with as
little tension as possible. The bobbin contains about 3600
meters of hollow fiber, and the thickness of hollow fiber
wound on the`bobbin is less than about 2 centimeters. The
bobbin is placed in a vessel containing tap water, and tap
water is added and purged from the ~essel for about 24 hours
at a rate of about 4 to 5 liters per hour per bobbi~ at
tap water temperatures C10 to 20C). The bobbin is then
stored for seven days in the vessel containing tap water
at about ambient temperature C20 to 25C). After storage,
the hollow fiber, while being maintained wet, is wound on
a skeiner to form hanks of holl~w fiber about 3 meters in
length. The winding of the hollow fiber on the skeiner is
conductet using the minimum tenqion required to effect the
windi~g. The hanks of hollow fi~er are hung vertically with
the b~res of the hollow fibers open at the bottom and are
allowed to dry for at least about thxee days at ambient
laboratory conditions Cabout 20-25C and 50 percent relative
humidity~. The dried hollow fiber has an outside diameter
of about 390 microns and an inside diameter of about 14Q
microns and appears, under scanning electron microscopy,
to resemble the hollow fiber depicted in Figure 1,
Test loops of ten hollow fiber each of about 15
centimeters in length are prepared. At one end, ~he test
loop is embedded in a tube sheet through which the bores
of the hollow fibers communicate. The other end is plugged.
The bores of the hollow fibers in each test loop are
subjected to a vacuum (about 50 to 100 millimeters of
` - -
510
-60- 07-0433
mercury absolute pressure~ for about-10 minutes, Each of
the test loops is immersed in a coating solution of about
2 weight percent post-curable polydimethylsiloxane (Sylgard
184 obtainable from Dow Corning Corp.) in isopenta~e while
maintaining the vacuum for about 10 minutes and then removed
from the coating solution. The vacuum is contained for
about 10 minutes after the test loop is removed from the
coating solution, and the test loops are dried at ambient
laboratory conditions (20 to 25C, about 40 to 60 percent
relative humidity). The permeabilities of the test loops
to hytrogen and methane are determined using essentially
pure gases at ambient laboratory temperatuxe Cabout 20 to
25C) with the exterior, feed surface side Cshell side)
of the hollow fibers being at a pressure of about 70
atmospheres absolute and the bore side of the hollow fiber
being at about 1 atmosphere absolute. The determined
permeabilities generally vary somewhat from test loop to
test loop, and some variations may be due to leaks in the
tube sheet, damage to the hollow fiber due to handling, or
the like. The average hydrogen permeability often ranges
~r~m about 75 to 150 GPU Clx106 cc~STP)/cm2-sec-cmHg) and
the average methane permeability often ranges from about
1 to 2 GPU. The ratio of the permeabilities Cseparation
factor) for hydrogen over methane is often about 50 to 60.
The failure pressure of the hollow fiber is determined to
be over llO kilograms per square centimeter C1600 pounds
per square inch~.
~14~510
-61- 07~043
EXAMPLE 2
A degassed polymer solution containing 32 weight
percent polysulfone P-3500 and a liquid carrier consisting
of l-formylpiperidine and formamide is spun through a
spinnerette (jet) of the type described in Example 1 into
a hollow fiber membrane in accordance with the procedure
9 et forth in Table I. The polymer solution (dope)
temperature for runs 885-1, 533-1-7, and 537-1 is not
directly measured and may be 60 to 70C. Deionized
water is employed as the injection fluid and is provided
at a rate sufficient to maintain the inside diameter of
the hollow fiber. The coagulation bath comprises water,
and the concentration of l-formylpiperidine in the
coagulation bath is maintained below about one percent
L5 by weight through purging. The hollow fiber is immersed
in the coagulation bath for a distance of about 1 meter.
The residence t~me of the hollow fiber in each of the
three wash baths (first, second and third godet baths~ is
as follows
Approxlmate SpirLnlng Speed, Approximate Resldence Time
Meters per Minute in Each Wash Bath Seconds
30.5 56.9
36.6 47 4
2538.1 45.5
~2.7 40.6
The hollow fiber is wound on a bobbln while being maintained
wet/ and the bobbin is washed in cold running tap water for
about 16 to 24 hours, then stored in a bucket filled with
water for seven days. The hollow fiber is then dried under
ambient laboratory conditions (22 to 25C and about 40 to
60 percent relative humidity), and formed into test loops,
coated and tested as described in connection wLth Example 1
except that a 1 weight percent Sylgard 184 solution is
employed and twenty holl~w fibers are used in the test
4~ 51~
-6~- 07-043.3
loop. The observed performance of the hollow fiber is
provided in Table II.
510.
-63- 07-0433
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510
- 64- 07-0433
_
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4~ 510
-65- 07-0433
EXAMPLE 3
The procedure of Example 2 is substantially repeated
except as follows. The degassed polymer solution consists
essentially of 35 weight percent polysulfone P-3500 and 64
weight percent l-formylpiperidine, the spi~nerette has an
outside diameter of 533 microns, an inner diameter of 203
microns and an injection capillary diameter of 127 microns.
The spinnerette is positioned about 10.2 centimeters above
the liquid level in the coagulation bath, the polymer
solution feed rate to the spinnerette is about 11.8 cubic
centimeters per minuteJ and the spinning speed is about 42 7
meters per minute. The temperature of the polymer solution
extruded from the spinnerette is a~out 54C. The coagulation
bath is at a temperature of about 1 to 2C; the first godet
bath, abaut 1 to 2C, and the second and third godet baths,
about 20C. The re~idence time of the hollow fiber in each
of the godet baths is about 40.6 seconds The hollow fiber
has an outside diameter of 445 microns and an inside
diameter of 166 microns prior to drying. A scanning electron
microscope photograph of a dried hollow fiber in accordance
with thiq Example is depicted in Figure 2. The collapse
pres3ure exhibited by the hollow fiber is about 170 kilograms
per square centimeter. The holl~w fiber is formed into test
loops, coated (except using a 2 weight percent Sylgard 184
solution) and tested as described in connection with Example 1
The hydrogen and methane permeabilities of the uncoated
hollow fiber are about 39 GPU ant 2.1 GPU. A similarly
prepared hollow fiber except that the temperature of the
coagulation bath is about 5C, has an inside diameter (wet)
of about 190 microns, and exhibits a collapse pressure of
about 100 to 150 kilograms per square centimeter and, when
coated and in test loops, a hydrogen permeability of about
100 GPU and a methane permeability of about 3.3 GPU.
1~4~510
-66- 07-0433
EXAMPLE 4
The procedure of Example 2 is substantially repeated
except as follows. A degassed polymer solution of 36 weight
percent polysulfone P-3500 and 64 weight percent of a
mixture of 87 parts by weight of l-formylpiperidine and 13
parts by weight of formamide is spun at a temperature of
about 52C through a-spinnerette having an outside diameter
of 457 microns, an inside diameter of 127 microns and an
injection capillary diameter of 76 microns at a rate of 5
cubic centimeters per minute. The spinnerette is positioned
10.2 centimeters from the liquid level in the coagulation
bath. The spinning speed is about 30.5 meters per minute.
The coagulation bath and first godet baths are at about 3OC,
and the sec~nd and third godet baths are at about room
temperature. The residence time in each of the three godet
baths is about 56.9 seconds. The hollow fiber is stored in
water in a storage buc~et for 1, 2 and 24 days,instead of
7 days, and samples of the hollow fibers are analyzed for
hytrogen and methane permeabilities, collapse pressures,
and solvent content after each of the storage periods. The
hytrogen and methane permeabilities of a test loop containing
the sample of ~he uncoated hollow fiber are determined for
the hollow fiber stored one day using a shell side pressure
of about 7.8 atmospheres absolute and a bore pressure of
about 4.4 atmospheres absolute and for the coated hollow
fibers stored for 1, 2 and 24 days using a shell side
pressure of about 70 atmospheres absolute and a bore
pressure of about 1 atmosphere absolute. The hollow fiber,
when dry, has an outside diameter of 456 microns and an
inside diameter of 188 microns. The results are provided
in Table III.
510
-67- 07-043
TABIE III
Days in Storage Bucket: 1 2 24
U~coatet ~2 457
Permeabilityf GPU
U~coated CH4 167 - -
Permeability, GPU
Coated a2 93 102 114
Permeability, GPU
Coated C~4 2.8 3.3 1.9
Permeability, GPU
Collapse Pressure, 140 140 170
kg/cm2
l-formylpiperidine in 2.25 1.46 0.95
Fiber, Weight %
510
-68- 07-0433
EXAMP~E 5
The procedure of Example 4 is substantially repeated
except tha~ the spinning rate is about 45.7 meters per
minute and the residence time in each of the gadet baths
is about 38 seconds. In one run, a chimney is positioned
between the spinnerette and the liquid level in the
coagulation bath such that a saturated atmosphere exists
within the chimney. The spinnerette is positioned 10.2
centimeters from the liquid level in the coagulation bath.
Another run is conducted which is substantially identical
except that no chimney is employed and the spinnerette
is pasitioned 5,1 centimeters above the liquid level in
the coagulation bath. Hollow fiber samples are formed
into test loops, coated and hydrogen and methane
permeabilities determined after 7 days storage in water~
The results are as follows:
No Chimney
~2 Permeability, GPU 129 123
CH4 Permeability, GPU 1.5 1.3
Outside D~ameter, microns 370 364
l~side Diameter, micron~ . 171 165
~141510
-69- 07-0433
EXAMPLE 6
The procedure of Example 2 is substantially repeated
except as follows. A degassed polymer solution consisting
of 38 weight percent polysulfone P-3500 and 62 weight percent
liquid carrier of 87 parts by weight l-formylpiperidine and
13 parts by weight formamide is spun at about 70C through
a spinnerette ha~ing an outside diameter of 635 microns,
an inside diameter of 228 microns, and an injection capillary
diameter of 152 microns at a rate of 6 cubic centimeters
per minute, The spinnerette is positioned about 2.5
centimeters abo~e the liquid level in the coagulation bath,
and the spinning rate is about 30,5 meters per minute. The
coagulation bath and first godet bath are at about 3C and
the seconds and third godet baths are at about room
temperature, The resit7ence times of the holl~w fiber in
each of the three godet baths is about 56,9 seconds. The
coagulation ~ath contains 7.5 weight percent acetic acid,
The dried hollow fibers ha,ve an outside diameter of 444
microns and an inside diameter of 171 microns. The hollow
fiber exhibits a collapse pressure of about 180 kilograms
per square centimeter, Samples of the hollow fiber are
fabricated into test loops and hydrogen and methane
permeabilities are determined both before and after coating,
CThe uncoated permeabilities are determined at a shell
pressure of about 7,8 atmospheres absolute and a bore
pressure of about 1 atmosphere absolute), The results
are as follows:
Uncoated Coated
H2 Permeability, GPU 409 136
CH4 Permeability, GPU 107 2.1
SiO
- 7~ 07-0433
EXAMPLE 7
The procedure of Example 6 is substantially repeated
except that the spinnerette has an outside di~meter of
483 microns; an inside diameter of 152 microns, and an
injection capillary diameter of 76 microns, and the hollow
fibers are stored in a buc~et under different conditions
as set forth in Table IV.
3s
1~41510
- 71- 07-04
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-72- 07-0433
EXAMPLE 8
The procedure of Example 2 is substa~tially repeated
except as follows. The polymer solution contains 32 weight
percent polysulfone P-3500 and 68 weight of a liquid
carrier of 85 parts by weight of l-formylpiperidine and
15 parts by weight of ethylene glycol and is spun at about
70C through a spinnerette having an outside diameter of
635 microns, an inside diameter of 229 microns, and an
injection capillary diameter of 152 microns at a rate of
7.2 cubic centimeters per minute. The spinning rate is
about 42.7 meter3 per minute, The spinnerette is about
10.2 centimeters from the liquid level in the coagulation
bath. The coagulation bath and ~irst godet baths are at
about 3C, and the second and third godet baths are at about
room temperature. The residence time of the hollow fiber in
each of the three godet baths is about 40.6 seconds. The
outside diameter of the hollow fiber is about 456 microns
and the inside diameter is about 205 microns. The hydrogen
permeability of the coated hollow fiber is about 64 GPU
and the carbon monoxide permeabillty is about 0.65 GPU.
4~510
-73- 07-0433
EXAMPLE 9
The procedure of Example 2 is substantially repeated
except as follows. The polymer solution contains 32
weight percent polysulfone P-3500 and 68 weight percent
liquid carrier ha~ing 90 parts by weight l-formylpiperidine,
5 parts by weight ethylene glycol, and 5 parts by weight
water. The polymer solution is spun at 65-70C through a
spinnerette having an outside diameter of 508 microns, an
inside diameter of 152 microns, and an injection capillary
diameter of 102 microns. The spinning rate is 43.5 meters
per minute. The spinnerette is about L7 centimeters from
the liquid level in the coagulation bath. The coagulation
bath ant first godet bath are at about 5C and the second
and third godet baths are at about room temperature. The
residence time of the hollow fiber in each of the three
godet baths is about 38 seconds. The outside diameter of
the hollow fiber is about 450 microns and the inside
diameter is about 236 microns. The hydrogen permeability
of thè coated hollow fiber is about 100 GPU, and the
carbon monoxide permeability is about 2.3 GPU. The
permeabilities are determined using a gas mixture of about
25 mole percent hydrogen and 75 mole percent carbon monoxide
using a shell side pressure of about 150 cent;meters of
mercury absolute aad laboratory vacuum on the bore side
of the hollow fibers.
4~ 510
-74- 07-0433
EXAMPLE 10
The procedure of Ex~mple 2 is substantially repeated
except as follows. The polymer solution is prepared
32 weight percent polysulfone P-3500 and 68 weight percent
liquid carrier ha~ing 80 parts by weight l-formylpiperidine,
10 parts by weight ethylene glycol, and 10 parts by weight
acetone. The concentration of the acetone is not determined
after degassing. The polymer solution is spun at about 80C
through a spinnerette having an outside diameter of 635
microns, an inside diameter of 228 microns, and an injection
capillary diameter of 152 microns. The spinning rate is
26.2 meters per minute. The spinnerette is about 20.3
centimeters from the liquid level in the coagulation bath.
The coagulation bath and first godet bath are at abaut 0C
and the second and third godet baths are at about 14C.
The residence time of the hollow fiber in each of the three
godet baths is about 66 seconds. The bore of the hollow
fiber is off-center and the outside diameter of the hollow
fiber (wet) is about 342 microns and the inside diameter
is about 165 microns, The hydrogen permeability of the
uncoated hollow fiber is about 2'25 GPU, and after coating,
about 82 GPU, and the met~ane permeability of the uncoated
hollow fiber is about 50 GPU, and after coating, about 1.6
GPU.
510
-75- 07-043
EXAMPLE 11
The procedure of Example 2 is substantially repeated
except as follows. The polymer solution contains 40 weight
percent polysulfone P-1700 ~a lower molecular weight
polysulfone but otherwise similar to P-3500 available from
Union Carbide Company) and 60 weight percent of a liquid
carrier having 87 parts by weight l-formylpiperidine and 13
parts by weight formamide. The polymer solution is spun at
about 70C through a spinnerette having an outside diameter
of 635 microns, an inside di meter of 229 microns, and an
injection capi~lary diameter of 152 microns at 9 cubic
centimeters per minute. The spinnerette is about 7.6
centimeters from the liquid level in t~e coagulation bath.
The spinning rate is about 30.5 meters per minute. The
coagulation bath and first godet bath are at about 4C, and
the second and third godet baths are at about room
temperature. The residence time of the hollow fiber in
each o the three godet baths is about 56.9 seconds. Tke
outside diameter of the hollow fiber is about 439 microns
and the inside diameter is about 154 microns. The collapse
pressure exhibited by the hollow fiber is about 175
kilo,grams per,squar,e centimeter, Samples of the hollow
fibers are stored in water for 7 or 22 or 23 days, drled
ant assembled into test loops. The test loops are treat~d
' in several manners. Some test loops prior to coating, are
flushed with isopentane. In thîs procedure, the bores of
the hollow fibers are subjected to a vacuum for about 10
minutes, immersed in isopentane at roo~ temperature Cabout
22 to 25C) for 10 minutes while maintaining the vacuum
on the bore side of the ~ollow fibers, and then removed
from the isopentane and the vacuum is maintained for an
additional 10 minutes. Also, test loops Ccoated and
uncoated~ are suspended in a closed container of gaseous
ammonia at room temperature and at a pressure of about
18 kilograms per square centimeter. Both coated and
uncoated hoLlow fibers are subjected to this ammonia
treatment. The permeabilities of the hollow fiber for
,
510
-7~ 07-0433
hydrogen and methane are determined using different shell
side pressures. The bore side pressure is about 1
atmosphere absolute, The results are provided in Table V.
5:~0
-77- 07-0433
}-~ ~ '
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~415~0
-78- 07-0433
EXAMPLE 12
The procedure of Example 2 is substantially repeated
except as follows. The polymer solution contains 34 weight
percent polysulfone P-3500 and 66 weight percent of a liquid
carrier comprising a mixture of 42.4 grams of substantially
anhydrous lithium chloride per 1000 milliliters of solution
with l-formylpiperidine. The polymer solu~ion is spun at
about 70 to 80C through a spinnerette having an outside
diameter of 635 microns, an inside diameter of 228 microns,
and an injection capillary diameter of 152 microns. The
spinnerette is 3.5 centimeters above the liquid level in the
coagulation bath. The spinning rate is about 20 meters per
minute. The coagulation bath is at 5C, the first godet
bath is at 5Cj and the second and third godet baths are at
about room temperature. The residence time in each of the
three godet baths is about 86 seconds. The outside diameter
of the hollow fiber is about 685 microns, and the inside
diameter is about 380 microns. The permeabilities are
determined using the procedure described in Example 9. The
coated hydrogen permeability is about 80 GPU; and the coated
carbon monoxide permeability is about 2.9 GPU. The hollow
fiber has a fine, nodular structure as observed from scanning
electron microscope photographs and exhibits an internal
surface area of about 75 s~uare meters per gram using a
B.E.T. analysis.
4~ 510
-79- 07-0433
EXAMPLE' 13
The procedure of Example 2 is substantially repeated
except as follows. The degassed polymer solution contained
32 weight percent poly(phenylene ether) sulfone available
from ICI, Ltd., Great Britain and 68 weight.percent of a
liquid carrier consisting, of 90 weight percent l-formyl-
piperidine and 10 weight percent formamide. The spinnerette
dimensions are outside diameter, 533 microns; inside dia-
meter, 203 microns, and the injection capillary diameter,
134 microns. The polymer solution rate through the spinnerette
is about 7.1 cubic centimeters per minute and the take-up
rate is about 42.4 meters per minute. The coagulation and
first wash baths are at about 1 to 2C and the second and
third wash baths are at about 19C. The approxlmate
polymer solution temperature is var~ed from about 40 to
55C and the dimension of the hollow fiber membranes before
drying are out9ide diameter about 435 microns and.inside
diameter about 150 microns; however, one sample has an oval
bore con~iguration. The observed performances of the hollow
fiber membrane9 are provided in TabLe VI.
TABLE VI
Approximate Permeabilities, GPU
Run ~o.Dope Temperature CUncoated (7.8) Coated (68)
H2 CH4 H2 CH4
1 55 325 120 41 1.3
2 50 400 140 47 1.0
30 3 40 170 50 35 0.3
*All permeabllities determinet with a bore side pressure at about
one atmosphere absolute. The shell side pressures in atmospheres
absolute are provided parenthetically.
4~ 510
-80- 07-0433
EXAMPLE 14
The procedure of Example 2 is substantially repeated
except as follows. The degassed polymer solution contains
32 weight percent polysulfone (P-3500) and 68 weight percent
of a liquid carrier consisting of about 90 weight percent
l-acetylpiperidine and 10 weight percent formamide. The
spinneret~e dimensions and outside diameter, 584 microns;
inside diameter, 178 microns; and injection capillary
diameter, 101 microns. The spinnerette is positioned about
5 centimeters above the le~eI of the liquid coagulant and
~ is at a temperature of abou~ 57C. The take-up rate is
about 42.4 meters per minute. The coagulation and first
wash baths are at about 2C and the second and third wash
baths are at about 16C. Further details are provided in
Table VII.
510
-81- 07-0433
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~4~510
-82- 07-0433
- EXAMPLE 15
A hollow fiber is prepared in accordance with the
procedure substantially set forth in Run No. 533-1 of
Example 2 e~cept that the hollow fiber is stored in water for
over seven days. Samples of the hollow fiber are dried in
a temperature and humidity controlled atmosphere (air) under
different conditions and then fabricated into a test loop.
The hydrogen and methane permeabilities of coated and
uncoated hollow fiber are determined. The results are
L0 provided in Table VIII.
510
-~3-. 07-0433
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1~4~510
-84- 07~0433
EXAMPLE 16
The procedure of Example 1 is substantially repeated
except as follows The polysulfone is dried for about 2
days at atmospheric pressure under a forced, dry air
atmosphere. The deaeration of the polymer solution is
conducted at atmospheric pressure under an atmosphere of
dry air. The polymer solution is extruded through the
spinnerette at about 8.1 cubic centimeters per minute
and the take up speed is about 42,4 meters per minute.
$he outside diameter of the wet hollow fiber is about 450
microns and the inside diameter is about 155 microns.
When the hanks of hollow fibers are hung to dry, the
hollow fibers remain looped, i.e., are not open at the
bottom. The dried hollow fiber mem~ranes have an outside
dizmeter of about 400 microns and an inside diameter of
about 140 microns,
1~415~0
-85- 07-0433
EXAMPLE 17
The procedure of Example 12 is substantially repeated
except as follows. The polymer solution contains 34 weight
percent of a suspension polymerized copolymer ~f 33 weight
S percent acrylonitrile and 67 weight percent styrene in 66
weigh~ percent of a liquid carrier containing 75 weight percent
l-formylpiperidine and 25 weight percent formamide. The
spinnerette is about 0.6 centimeter above the level of the
liquid coagulant, and the take-up rate is about 13 to 14
meters per minute. The coagulation bath and first wash bath
are at about 2 to 3C and the second and third wash baths
are at about 17C. The hollo~ fibers are dried and dipped in
methanol or pentane and then exposed to achieve vapor at low
partial pressure. The hollow fi~er membranes are then coated
with a solution of about 3 weight percent sylgard in pentane.
The coated hydrogen permeability is about 20 to 40 GPU, and
the separation factor for hydrogen over carbon monoxide is
about 20 to 40.
510
-86- 07-0433
EXAMPLE 18
The procedure of Example 12 is substantially repeated
except as follows. The polymer solution contains 30 weight
percent of a m~.thyl bromi~ated poly(phenylene oxide) (about
45 mole percent brominated) i~ 70 weight percent
l-formylpiperidine. The spinnerette is about 2.6 centimeters
above the level of the liquid coagulant, and the take-up
rate is about 13 to 14 meters per minute. The coagulation
and first wash baths are at about 46 to 48C and the second
and third wash baths are at about 14C. When coated, the
hollow fiber membranes exhibit a hydrogen permeability of
about 90 to 100 GPU and a separation factor for hydrogen over
carbon monoxide of about 13.
