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BACKGROUND AND OBJECTS OF THE INVENTION
This invention relates to a process for preparing electrically conductive
shaped articles such as films or fibrous material from a
polybenzimidazole, and to the conductive shaped articles produced thereby.
The invention further relates to an electrically conductive composite
comprising electrically conductive polybenzimidazole fibrous material and
to a process for preparing such material. The invention is useful for EMI
shielding and static dissipation, in forming electrically conductive
resins and paints, and as a membrane in certain gas or liquid separations.
It is known in the art to plate copper metal onto various polymeric
substrates. For example, Davis U.S. Pat. No. 4,143,186 discloses a process
for electroless plating of copper on substrates such as polyimides and
polyparabanic acids (polyimidazoletriones) comprising deposition of copper
in an acidic bath containing a divalent copper salt such as copper sulfate
and a reducing agent such as dimethyl amine borane, wherein the copper
salts may be present in a concentration of between 0.05 and 0.15M.
Further, McCormack et al U.S. Pat. No. 4,301,196 discloses a process for
electroless plating of copper by employing a bath comprising copper ions,
a reducing agent, a pH adjustor, and a depolarizing agent. The
depolarizing agent can be 1,3-imidazole, benzimidazole, and the like
(Column 4, lines 24-45). The process may be employed to electrolessly
deposit copper on paper, glass, or synthetic resins and plastics including
nylons, acrylic, Mylar.RTM. polyester film, and epoxies (Column 7, lines
31-136). See also, U.S. Pat. Nos. 3,736,170; 3,993,801; 4,199,623;
4,209,331; and 4,261,800, which disclose various processes for electroless
deposition of copper onto polymeric substrates.
U.S. Pat. No. 4,374,893 discloses the preparation of textiles based upon
synthetic polymers such as polyesters and polyamides, having a surface
coating of at least 3 percent copper sulfide, the composition of which is
such that the atomic ratio Cu/S is between 1.5 and 2, preferably more than
1.7. The copper sulfide is formed by a process comprising treatment with
hydrogen sulfide, followed by contact with at least one reducing agent and
copper cations provided by a copper salt.
It is also known to form metal salt complexes of alkylbenzimidazole
polymers wherein such salts may be prepared by mixing dimethylsulfoxide
solutions of an inorganic metal salt and subsequently stripping off the
DMSO solvent such that metal is complexed within the benzimidazole rings.
Suitable metal cations include Cu(II), Zn(II), and Cd(II). The electrical
resistivity of the resultant salt may be increased by a factor of twenty
or more relative to the neutral polymer. [Aharoni et al, "Electrical
Resistivities and ESCA Studies on Neutral Poly(alkylbenzimidazole), Their
Salts and Complexes," Journal of Applied Polymer Science, Volume 23,
2363-2660 (1979)].
It is therefore an object of the present invention to provide a process for
preparing an improved highly electrically conductive shaped article such
as a film or fibrous material.
It is a still further object of the present invention to provide a process
for preparing a improved electrically conductive shaped article which is
flexible and ductile.
It is a further object of the present invention to provide an improved
electrically conductive shaped article having high levels of covellite
copper sulfide in association therewith, wherein the copper sulfide is
substantially entirely in the form of covellite copper sulfide.
It is a still further object of the invention to provide a process for
preparing a composite article which incorporates an improved electrically
conductive fibrous material which is heat stable and which may be
processed in a molten resinous matrix without destruction of the fibrous
material.
It is a still further object of the invention to provide an electrically
conductive monolithic composite incorporating an improved electrically
conductive fibrous material.
It is a still further object of the invention to provide an electrically
conductive polymer composition incorporating an improved electrically
conductive fibrous material.
It is a still further object of the invention to produce conductive shaped
articles which are suitable for use in EMI shielding applications and
other applications where electrically conductive media are desired.
It is a still further object of the invention to produce a conductive film
which is suitable for use in gas or liquid separators.
It is another object of the invention to produce patterned conductor film
or sheet material for use as heating elements or in electronic circuitry
such as printed circuit boards.
These and other objects, as well as the scope, nature, and utilization of
the claimed invention will be apparent to those skilled in the art by the
following detailed description and appended claims.
SUMMARY OF THE INVENTION
According to the present invention, an electrically conductive shaped
article is prepared from a polybenzimidazole shaped article by:
(a) contacting the polybenzimidazole shaped article with a source of
cuprous ions to produce a cuprous ion-impregnated article;
(b) contacting the resulting cuprous ion-impregnated polybenzimidazole
shaped article with a sulfiding agent capable of sulfiding the cuprous
ions to form electrically conductive covellite copper sulfide in
association with said polybenzimidazole; and, optionally,
(c) washing the resulting electrically conductive polybenzimidazole shaped
article to substantially remove residual reactants adhering thereto.
In a preferred embodiment, an electrically conductive shaped article is
prepared from a polybenzimidazole shaped article by:
(a) cuprous ion-impregnating the polybenzimidazole shaped article with an
aqueous solution to which was added a concentration of approximately 0.25
to 10 weight percent of copper ions, added as copper sulfate, and between
about 0.5 and 10 weight percent of an hydroxylamine reducing agent, while
at a temperature Of between about 80.degree. C. and about 105.degree. C.
for between about 15 minutes and about 2 hours;
(b) subjecting the resulting cuprous ion-impregnated polybenzimidazole
shaped article to a sulfiding treatment in a solution comprising a
thiosulfate sulfiding agent in a concentration of approximately 5 to 15
percent by weight while at a temperature of between about 90.degree. C.
and about 105.degree. C. for an additional period of time between about 15
minutes and about 2 hours effective to produce an electrically conductive
shaped article having covellite copper sulfide in association therewith;
and
(c) washing the resulting electrically conductive shaped article to
substantially remove residual reactants adhering thereto.
In another aspect of the invention, an electrically conductive fibrous
material is provided which comprises polybenzimidazole fibrous material in
association with approximately 5 to 60 percent, and preferably 35 to 60
percent, by weight of covellite copper sulfide, based upon the total
weight of the product.
In yet another aspect of the invention, electrically conductive film and
fibrous material are provided which comprises polybenzimidazole film
material and fibrous material, respectively, in association with
approximately 5 to 60 percent by weight of covellite copper sulfide, based
upon the total weight of the product.
In still another aspect of the invention, an electrically conductive
composite article is prepared by a process comprising the steps of:
(a) cuprous ion-impregnating a polybenzimidazole fibrous material with a
solution of a cupric salt and a reducing agent capable of reducing cupric
ions to cuprous ions;
(b) subjecting the resulting cuprous ion-impregnated fibrous material to a
sulfiding treatment in a solution comprising a sulfiding agent capable of
sulfiding said cuprous ions to covellite copper sulfide so as to produce
electrically conductive polybenzimidazole fibrous material;
(c) washing the resulting electrically conductive polybenzimidazole fibrous
material to remove residual reactants adhering thereto; and
(d) surrounding the resulting electrically conductive fibrous material with
a substantially continuous polymeric matrix to produce a monolithic
electrically conductive composite article.
In still another aspect of the invention, a monolithic electrically
conductive composite article is provided which comprises electrically
conductive polybenzimidazole fibrous material in association with
approximately 5 to 60 percent by weight of covellite copper sulfide based
upon the total weight of the conductive fibrous product, incorporated
within a substantially continuous polymeric matrix.
In another aspect of the invention, a monolithic electrically conductive
article comprising a fabric, paper, or felt is provided which includes
polybenzimidazole fibrous material in association with approximately 5 to
60 weight percent of covellite copper sulfide, the fabric, paper, or felt
being incorporated within a solid, continuous, polymeric matrix.
In yet another aspect of the invention, a polymer composition suitable for
use in electrically conductive end uses is provided, comprising
electrically conductive polybenzimidazole fibrous material in association
with approximately 5 to 60 weight percent of covellite copper sulfide and
a polymeric carrier.
In another aspect of the invention, a monolithic electrically conductive
composite article is provided, comprising polybenzimidazole fibrous
material in association with approximately 35 to 60 percent by weight of
covellite copper sulfide incorporated within a solid, continuous, cured
epoxy resin matrix.
In another aspect of the present invention, a sheetlike article is
provided, comprising polybenzimidazole fibrous material in association
with from about 3 to about 60 percent by weight of electrically conductive
covellite copper sulfide, the sulfide being present in at least one layer
comprising a multiplicity of the fibers and having a thickness of
approximately 1 mil to 1 inch, the article having a sheet resistivity of
from about 1 to about 1000 ohms/sheet.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a transmission electron micrograph (10,000X) of a thin
cross-section of a sample of conductive bilobal polybenzimidazole fiber
associated with covellite copper sulfide.
FIG. 2 is a transmission electron micrograph (100,000X) of a thin
cross-section of a sample of conductive bilobal polybenzimidazole fiber
associated with covellite copper sulfide, showing a contiguous surface
coating of the copper sulfide with considerable penetration.
FIG. 3 is a graph of the resistance variation with temperature of an
electrically conductive thermally stabilized fibrous material having
covellite copper sulfide associated therewith produced by procedures
similar to those of Example I.
FIG. 4 is a transmission electron micrograph (4000X) of polybenzimidazole
fiber after one hour treatment with cuprous ions.
FIG. 5 is an X-ray map for copper of polybenzimidazole fiber after one hour
treatment with cuprous ions.
FIG. 6 is a transmission electron micrograph (2000X) of polybenzimidazole
fiber after sulfiding for one hour.
FIG. 7 is an X-ray map for copper of polybenzimidazole fiber after one hour
of sulfiding.
FIG. 8 is a set of X-ray diffraction patterns of the electrically
conductive polybenzimidazole fibrous material produced in accordance with
the procedure of Example II, showing the covellite copper sulfide phase in
a Debye-Scherrer pattern.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The shaped article which is rendered electrically conductive in accordance
with the present invention is a preformed polybenzimidazole material which
can be produced in various forms, e.g. solid thin films, flat films,
porous, microporous, or semi-permeable membranes, solid or hollow fibers
and the like, by methods previously known in the art. For example, the
shaped article can be a fibrous material in the form of staple yarns,
continuous filament yarns, multifilamentary tows, tapes, strands, cables,
fibrils, fibrids, papers, woven fabrics, nonwoven fabrics, and the like.
The linear polybenzimidazoles which are used as the starting material in
the present invention are a known class of heterocyclic polymers. Typical
polymers of this class and their preparation are more fully described in
U.S. Pat. No. 2,895,948, U.S. Pat. No. 26,065, and in the Journal of
Polymer Science, Vol. 50, pages 511-539 (1961), the contents of which are
herein incorporated by reference. The polybenzimidazoles comprise
recurring units selected from the group consisting of units of the
following Formulas I and II, and preferably consist essentially of such
recurring units. Formula I is:
##STR1##
wherein R is a tetravalent aromatic nucleus, preferably asymmetrically
substituted, with the nitrogen atoms forming the benzimidazole rings being
paired upon adjacent carbon atoms, i.e., ortho carbon atoms, of the
aromatic nucleus, and R' is a member of the group consisting of (1) an
aromatic ring, (2) an alkylene group (preferably those having 4 to 8
carbon atoms), and (3) a heterocyclic ring from the group consisting of
(a) pyridine, (b) pyrazine, (c) furan, (d) quinoline, (e) thiophene, and
(f) pyran.
Formula II is:
##STR2##
wherein Z is an aromatic nucleus having the nitrogen atoms forming the
benzimidazole ring paired upon adjacent carbon atoms of the aromatic
nucleus.
Preferably, aromatic polybenzimidazoles are selected, e.g., polymers
consisting essentially of the recurring units of Formulas I and II wherein
R' is at least one aromatic ring or a heterocyclic ring.
As set forth in U.S. Pat. No. 26,065, the aromatic polybenzimidazoles
having the recurring units of Formula II may be prepared by
self-condensing a trifunctional aromatic compound containing only a single
set of ortho disposed diamino substituents and an aromatic, preferably
phenyl, carboxylate ester substituent Exemplary of polymers of this type
is poly-2,5(6)-benzimidazole prepared by the autocondensation of phenyl
1,4-diaminobenzoate.
As set forth in the above-mentioned patent, the aromatic polybenzimidazoles
having the recurring units of Formula I may be prepared by condensing an
aromatic tetraamine compound containing a pair of ortho diamino
substituents on the aromatic nucleus with a dicarboxylic compound selected
from the class consisting of (a) the diphenyl ester of an aromatic
dicarboxylic acid, (b) the diphenyl ester of a heterocyclic dicarboxylic
acid wherein the carboxyl groups are substituents upon a carbon in a ring
compound selected from the group consisting of pyridine, pyrazine, furan,
quinoline, thiophene, and pyran and (c) an anhydride of an aromatic
dicarboxylic acid.
Examples of polybenzimidazoles which have the recurring structure of
Formula I are as follows:
poly-2,2'(m-phenylene)-5,5'-bibenzimidazole;
poly-2,2'-(pyridylene-3",5")-5,5'-bibenzimidazole;
poly-2,2'-(furylene-2",5")-5,5'-bibenzimidazole;
poly-2,2'-(naphthalene-1",6")-5,5'-bibenzimidazole;
poly-2,2'-(biphenylene-4",4")-5,5'-bibenzimidazole;
poly-2,2'-amylene-5,5'-bibenzimidazole;
poly-2,2'-octamethylene-5,5'-bibenzimidazole;
poly-2,6-(m-phenylene)-diimidazobenzene;
poly-2,2'-cyclohexeneyl-5,5'-bibenzimidazole;
poly-2,2'-(m-phenylene-5,5'-di(benzimidazole) sulfide;
poly-2,2'-(m-phenylene-5,5'-di(benzimidazole) sulfone;
poly-2,2'-(m-phenylene-5,5'-di(benzimidazole) ether;
poly-2,2'-(m-phenylene-5,5'-di(benzimidazole) methane;
poly-2'2,2'-(m-phenylene-5',5"-di(benzimidazole) propane-2,2';
and poly-2',2"(m-phenylene-5',5"-di(benzimidazole) ethylene-1,2,
where the double bonds of the ethylene groups are intact in the final
polymer.
Another polybenzimidazole that is suitable for use in the present process
is one prepared from poly-2,2'-(m-phenylene)-5,5'bibenzimidazole, the
recurring unit of which is:
##STR3##
Any polymerization process known to those skilled in the art may be
employed to prepare the polybenzimidazole which is utilized to form shaped
articles for use in the process of the present invention. See, e.g., U.S.
Pat. No. 3,669,038, the content of which is hereby incorporated by
reference. With respect to aromatic polybenzimidazoles, preferably,
equimolar quantities of the monomeric tetraamine and dicarboxyl compound
may be introduced into a first stage melt polymerization reaction zone and
heated therein at a temperature above about 300.degree. C., preferably at
least 250.degree. C., and more preferably from about 270.degree. C. to
300.degree. C. The reaction is conducted in a substantially oxygen-free
atmosphere, i.e., below about 20 p.p.m. oxygen and, preferably, below
about 8 p.p.m. oxygen, until a foamed prepolymer is formed. Usually, the
first stage reaction is continued until a prepolymer is formed having an
inherent viscosity, expressed as deciliters per gram, of at least 0.1, and
preferably from about 0.13 to 0.3 (determined from a solution of 0.4 grams
of the polymer in 100 ml. of 97 percent H.sub.2 SO.sub.4 at 25.degree.
C.).
After the conclusion of the first stage reaction, which normally takes at
least 0.5 hour and, preferably, 1 to 3 hours, the foamed prepolymer is
cooled and then powdered or pulverized in any convenient manner. The
resulting prepolymer powder is then introduced into a second stage
polymerization reaction zone wherein it is heated under substantially
oxygen-free conditions, as described above, to yield a polybenzimidazole
polymer product, desirably having an I.V., as measured above, of at least
0.4, e.g., 0.8 to 1.1 or more. When the polybenzimidazole polymer is to be
utilized in the form of a hollow fiber, its preferred inherent viscosity
is at least about 0.5, and, most preferably, in the range of from about
0.7 to about 1.4.
The temperature employed in the second stage is at least 250.degree. C.,
preferably, at least 325.degree. C., and, more preferably, from about
350.degree. to 425.degree. C. The second stage reaction generally takes at
least 0.5 hours, and, preferably, from about 1 to 4 hours or more. It is
of course also possible to prepare these polymers via a one-step reaction.
The solvents utilized to form the polybenzimidazole polymer solutions for
producing the shaped articles used in the process of the present invention
include those solvents which are commonly recognized as being capable of
dissolving the particular polybenzimidazole polymer. For instance, the
solvents may be selected from those commonly utilized in the formation of
polybenzimidazole dry spinning solutions. Illustrative examples of
suitable solvents include N,N-dimethylacetamide, N,N-dimethylformamide,
dimethyl sulfoxide, and N-methyl-2-pyrrolidone. The particularly preferred
solvent is N,N-dimethylacetamide. Additional representative solvents
include formic acid, acetic acid, and sulfuric acid.
The polymer solutions may be prepared, for example, by dissolving
sufficient polybenzimidazole in the solvent to yield a final solution
containing from about 5 to 30 percent by weight of polymer based on the
total weight of the solution, and preferably from about 10 to 20 percent
by weight.
The quantity of polybenzimidazole dissolved in the solvent should be such
that the resulting solution has a viscosity of about 50 to 4000 poises at
30.degree. C., and preferably about 400 to 600 poises.
One suitable means for dissolving the polymer in the solvent is by mixing
the materials at a temperature above the normal boiling point of the
solvent, for example, about 25.degree. to 120.degree. C. above such
boiling point, and at a pressure of 2 to 15 atmospheres for a period of 1
to 5 hours. The resulting solutions then preferably are filtered to remove
any undissolved polymer. A minor amount of an additive such as lithium
chloride optionally may be provided in the spinning solution in accordance
with the teachings of commonly assigned U.S. Pat. Nos. 3,502,606 and
4,321,182. In addition to lithium chloride, suitable additives include
zinc chloride, N-methyl morpholine, triethylamine and triethanolamine.
Also suitable are organolithium compounds selected from the group
consisting of RCO.sub.2 Li, RSO.sub.3 Li, ROSO.sub.3 Li, and mixtures
thereof, wherein R is a hydrocarbon radical having from 1 to about 50
carbon atoms. Representative lithium salts are lithium formate, lithium
acetate, lithium propionate, lithium butyrate, lithium isobutyrate,
lithium valerate, lithium cetylate, lithium stearate, etc. Representative
lithium hydrocarbon sulfonates are lithium lauryl sulfonate, lithium cetyl
sulfonate, etc. Representative lithium hydrocarbon sulfates are lithium
lauryl sulfate, lithium cetyl sulfate, etc. The preferred organolithium
compound is lithium stearate The additive serves the function of
preventing the polybenzimidazole polymer from phasing out of the solution
upon standing for extended periods of time.
The formation of the various shaped articles for use in the process of the
present invention may be conducted according to any of the suitable
methods known in the art. For example, in the preparation of a porous,
microporous or semipermeable membrane the solution of polybenzimidazole
polymer is deposited upon a support to form a wet film of the same. The
nature of the support is not critical and may be selected from a variety
of materials including ceramic, polymeric compositions, glass, or metallic
plates (e.g., stainless steel), or flexible, porous materials such as
woven or non-woven fabrics. Such fabrics can comprise fibers of materials
such as metals, inorganic compounds, minerals, glass and natural or
synthetic polymers. The support is preferably provided with retaining
sides, or raised edges, whereby the solution is confined to the surface
thereof at the desired location until its consistency is such that
retaining sides are no longer needed. Numerous techniques are available
for the application of the solution to the support as will be apparent to
those skilled in the art. For instance, the polybenzimidazole polymer
solution may be simply poured upon a level support in a quantity
sufficient for it to assume the desired uniform thickness. A blade
optionally may be drawn over the surface of the wet film to aid the
deposition of a wet film of uniform thickness. In a preferred embodiment
of the invention, the solution is deposited by the utilization of a doctor
blade caster. In a preferred embodiment of the invention, the solution is
deposited by the utilization of a doctor blade caster. Reverse roll
techniques and calender machines can also be employed. It is presently
preferred to apply the polybenzimidazole polymer solution to a flexible
porous support by doctor blade caster or reverse roll techniques.
The thickness of the wet film deposited upon the support is influenced by
the desired thickness of the polybenzimidazole semipermeable membrane
ultimately to be produced. Commonly the wet film is deposited upon the
support in a substantially uniform thickness of about 1 to 30 mils and
preferably 2 to 18 mils. In a particularly preferred embodiment of the
invention, the wet film is deposited in a thickness of about 4 to 8 mils.
A quantity of solvent is next evaporated from the exposed surface o the wet
film to allow the formation of a relatively thin solid layer (i.e., a thin
porous polymeric film) on the exposed surface of the same. The thin solid
film commonly exhibits a thickness of about 0.1 to 10 microns, and
preferably about 1 to 5 microns. During the formation of the solid layer
on the exposed surface of the film, the solvent present near the surface
of the wet film is flashed off and a thick coagulated solid layer or skin
of polybenzimidazole polymer remains. The remaining portion of wet film
which supports the solid layer remains essentially unchanged while the
solid layer is formed. The solid layer accordingly exhibits a density
which is substantially greater than that of the remaining portion of the
film which has not undergone coagulation and continues to possess a liquid
consistency.
The evaporation of solvent from the exposed surface of the wet film may be
accomplished by a variety of techniques, as will be apparent to those
skilled in the art. For instance, a stream of air or other gas at ambient
or at an elevated temperature (e.g., approaching the boiling point of the
solvent) may be simply directed at the exposed surface of the wet film.
Alternatively, the wet film may be simply allowed to stand in an
uncirculated gaseous environment wherein the requisite degree of solvent
evaporation is accomplished. In a further embodiment of the invention, the
gaseous atmosphere to which the wet film is exposed may be at reduced
pressure, e.g., 100 mm. Hg, up to near atmospheric pressure. It will be
apparent to those skilled in the art that the rate at which the solvent is
evaporated increases with the temperature of the gaseous atmosphere
impinging upon the wet film, the flow rate of the gaseous atmosphere, and
with reduced pressure. The time required to form the desired thin solid
layer upon the exposed surface of the wet film commonly ranges from about
5 seconds to 30 minutes, and preferably from about 15 seconds to 5
minutes. In a preferred embodiment of the invention, the wet film is
exposed to a stream of circulating air at ambient temperature (e.g., about
25.degree. C.) and pressure for about 1 to 5 minutes. When the air is not
circulated, longer exposure times advantageously may be employed.
The resulting film bearing a thin solid layer upon its surface is next
converted to a semipermeable membrane by washing the same with a
non-solvent for the polybenzimidazole polymer which is capable of removing
residual quantities of the polybenzimidazole solvent. During the wash
step, the remaining polybenzimidazole polymer within the wet film is
coagulated while the solvent which originally dissolved the same is
removed. The wash medium is preferably aqueous in nature, e.g., water
containing less than about 10 weight percent of organic solvents miscible
in water, and is most preferably water. Aqueous solutions of
polybenzimidazole solvents, such as N,N'-dimethylacetamides or polyhydroxy
aliphatic alcohols having from two to about six carbon atoms and two or
three hydroxy groups, can be used. Such alcohols can be used neat as a
preferred non-aqueous wash medium. The wash step is preferably carried out
by immersing the film in the wash medium Alternatively, any other
convenient means for contacting the film with the wash medium may be
utilized, such as by spraying the film with the same. In a preferred
embodiment of the invention a water wash medium is provided at a
relatively cool temperature, e.g., at about 5.degree. to 30.degree. C.,
and at a temperature of about 10.degree. to 25.degree. C. in a
particularly preferred embodiment. The time required to accomplish
coagulation of the remaining polybenzimidazole polymer and the
substantially complete removal of residual solvent for the same varies
with the temperature of the wash medium. Satisfactory wash times commonly
range from about 30 seconds to 20 minutes, and preferably about 2 to 5
minutes. Considerably longer wash times may be employed, but generally
with no commensurate advantage.
The resulting flat film membranes formed of the polybenzimidazole polymer
consist of an outer relatively thin surface layer formed during the
evaporation step adjacent to a relatively thick layer of a more porous
structure formed during the wash step.
Alternatively, the polybenzimidazole starting material may be formed into
solid or hollow fibers according to processes well known in the art These
fibers can be prepared by solution spinning using a dope of the
polybenzimidazole polymer. Suitable solvents for the preparation of this
dope include those solvents which are commonly recognized as being capable
of dissolving the particular polybenzimidazole polymer, such as those
solvents used in preparing the polymer solution for film preparation as
previously described. Particularly preferred is a solvent system
comprising N,N-dimethylacetamide and lithium chloride.
Using conventional equipment and techniques, the dope is placed in an
extrusion or spinning bomb at the desired solids content. The amount of
extrusion solids, of course, is dependent upon the viscosity and molecular
weight of the particular polybenzimidazole polymer used. However, using
N,N-dimethylacetamide and lithium chloride as a solvent system, solids
concentrations in the range of about 20 to 30 weight percent are typical.
In selecting the amount of solids to be used, it is generally desirable to
use a dope having the highest possible viscosity which can still be
extruded at the desired extrusion temperature. Extrusion temperatures
generally range from about room temperature or slightly lower to as high
as 150.degree. to 180.degree. C.
The bomb containing the spinning dope is attached to the spinnerette and
pressurized with sufficient pressure to cause the polymer solution
contained in the bomb to escape through the spinnerette jet It is, of
course, understood that in order to prepare optimum hollow fibers the dope
placed in the bomb should be filtered either prior to placing it in the
bomb or just prior to spinning. The spinnerette or nozzle through which
the hollow fibers are spun comprises an inner nozzle and a concentric
nozzle arranged about the inner nozzle and is referred to as a concentric
hollow jet spinnerette. In order to maintain the hollow configuration of
spun fibers a fluid, either gaseous or liquid, is forced through the inner
nozzle. Examples of this fluid include nitrogen and ethylene glycol.
As the polybenzimidazole polymer is spun it is fed into a coagulation bath,
which bath comprises a solvent or solvent system which is a non-solvent
for the polybenzimidazole polymer employed and preferably is a solvent for
the dope solvent. Though the hollow fiber can be spun directly into the
coagulation bath, it is preferred to expose the spun fiber to a gas
capable of effecting surface coagulation or drying of the fiber.
Generally, this can be accomplished by spinning the fiber into air for
usually not more than 1 to 10 seconds, or in any case no longer than is
required to coagulate a thin surface layer on the fiber. The preferred
types of coagulation bath solvents include water, ethylene glycol and
mixtures of these two. The speed at which the hollow filament is
introduced into the coagulation bat can vary depending upon the length of
the bath used. Generally, speeds of about 2 to 50 meters per minute,
preferably 5 to 28 meters per minute, are utilized with baths which are
from 1 to 10 meters, preferably 1 to 5 meters, in length. Thus exposure
to the bath should be in the range of about 2 to 10 seconds or longer.
During the coagulation bath treatment step, the hollow filament material is
preferably subjected to a drawing operation. The purpose of this drawing
operation is to decrease the size of the hollow filament, thereby
increasing its surface area per unit volume as well as its strength.
Preferably, the spun hollow filament material is drawn at a ratio between
1:1 and 20:1, most preferably between 5:1 and 15:1. The resulting
filaments exhibit an inside diameter of about 12 microns to about 500
microns and an outside diameter of 25 microns to about 1000 microns,
preferably 25 to 250 microns and 50 to 500 microns respectively.
In a preferred embodiment the preformed polybenzimidazole semipermeable
membrane or other article can be chemically modified to form a covalently
bonded sulfonated polybenzimidazole material, as disclosed in U.S. patent
application Ser. No. 395,648, filed July 6, 1982, the content of which is
herein incorporated by reference. To effect this modification, the
preformed polybenzimidazole article is sulfonated by contacting the
article with SO.sub.3 or with any compound which releases SO.sub.3.
Suitable sulfonating agents include sulfuric acid, complexes of SO.sub.3
with a Lewis base or other organic compound, and covalent | | |
