Stable non-corrosive solution of polybenzimidazole suitable for use in the formation of shaped articles

An improved solution of a polybenzimidazole suitable for use in the formation of shaped articles (e.g. fibers, films, and the like) is provided which includes a minor amount of an organo-lithium compound (as defined) which has been found to be capable of inhibiting the separation of the solution into phases of greater and lesser concentrations of the polybenzimidazole upon the passage of time. In a preferred embodiment the organo-lithium compound is lithium stearate. Not only is stability imparted to the solution prior to the formation of a shaped article, but the potential for catastrophic corrosive stress cracking of stainless steel equipment in contact with the solution is eliminated such as that which may occur following the extended use of the common prior art additive, lithium chloride.

BACKGROUND OF THE INVENTION 
It is recognized that polybenzimidazoles and particularly aromatic 
polybenzimidazoles are characterized by a high degree of thermal 
stability. They may be shaped to form fibers, films and other articles of 
wide utility which show great resistance to degradation by heat, 
hydrolytic and oxidizing media. 
It has been further recognized that solutions of polybenzimidazoles in an 
organic solvent suitable for extrusion into shaped articles often cannot 
be stored much more than one to three days without a "phasing out" 
phenomenon occurring, i.e. a separation of the solution into two phases 
containing larger and smaller concentrations of polymer. The exact time of 
the onset of such phase out is unpredictable. This phased out solution is 
completely unsuitable for extrusion into shaped articles and interferes 
with routine storage of the polymer solution in an extrusion plant. While 
the phased out solution can generally be transformed into a homogeneous 
solution by means of additional mixing procedures, this results in 
additional expense and inconvenience. 
Commonly assigned U.S. Pat. No. 3,502,606 to Anthony B. Conciatori and 
Charles L. Smart discloses that this undesirable phase separation can be 
prevented if one incorporates a minor amount of certain additives in the 
solution of the polybenzimidazole. It is there contemplated that the 
additive be lithium chloride, zinc chloride, N-methyl morpholine, 
triethylamine, or triethanol amine. Heretofore lithium chloride has been 
the additive of choice because of its effectiveness in solving the phase 
separation phenomenon. 
However, it is well known that extended contact of stainless steel 
equipment (e.g. of common 300 series stainless steel) with halides (e.g., 
chlorides) has the tendency to lead to the catastrophic stress cracking of 
the stainless steel. For instance, it is common that various vessels for 
forming polymer solutions and pressure lines for the same be formed of 
such stainless steel. The onset of stress cracking can come without 
warning and necessitates a time consuming periodic visual inspection of 
the equipment. Should such stress cracks be observed additional 
replacement costs must be incurred as well as costly down time for the 
plant. Alternatively, if such procedures are to be eliminated the 
equipment heretofore must originally be constructed of more expensive 
alloys which are not as susceptible to corrosive stress cracking. This 
approach would lead to a significantly greater capital expenditure for a 
commercial plant. 
It is an object of the present invention to provide a stable solution of a 
polybenzimidazole suitable for use in the formation of shaped articles 
which does not rely on the inclusion of a lithium chloride additive as is 
common in the prior art. 
It is an object of the present invention to provide a highly stable 
solution of a polybenzimidazole suitable for use in the formation of 
shaped articles wherein the additive employed has no known propensity to 
promote corrosive stress cracks when contacted with common forms of 
stainless steel for extended periods of time. 
It is an object of the present invention to provide a highly stable 
solution of a polybenzimidazole which is non-corrosive to common 300 
series stainless steel thereby making possible significant capital savings 
during the construction and operation of a plant in which such solution is 
handled. 
It is another object of the present invention to provide a process for 
forming shaped polybenzimidazole articles (e.g. fibers, films, etc.) 
wherein neither phase out of the polymer solution involved nor the 
potential for corrosive attack of the equipment employed need be 
considered. 
It is a further object of the present invention to provide a process which 
is capable of forming polybenzimidazole fibers and films of unusually high 
surface area. 
These and other objects, as well as the scope, nature, and utilization of 
the invention will be apparent to those skilled in the art from the 
following detailed description and appended claims. 
SUMMARY OF THE INVENTION 
It has been found that a stable non-corrosive solution suitable for the 
formation of shaped polybenzimidazole articles comprises: 
(a) a solvent capable of dissolving the polybenzimidazole which is selected 
from the group consisting of N,N-dimethylacetamide, N,N-dimethylformamide, 
dimethylsulfoxide, and N-methyl-2-pyrrolidone, 
(b) a polybenzimidazole dissolved in the solvent, and 
(c) a minor amount of an organo-lithium compound dissolved in the solvent 
which is non-corrosive and is capable of inhibiting the separation of the 
solution into phases of greater and lesser concentrations of the 
polybenzimidazole upon the passage of time with the organo-lithium 
compound being selected from the group consisting of RCO.sub.2 Li, 
RSO.sub.3 Li, ROSO.sub.3 Li, and mixtures of any two or all three of the 
foregoing, wherein R is a hydrocarbon radical having 1 to 50 carbon atoms. 
It has been found that an improved process for forming shaped 
polybenzimidazole articles comprises: 
(a) dissolving a polybenzimidazole and a minor amount of an organo-lithium 
compound selected from the group consisting of RCO.sub.2 Li, RSO.sub.3 Li, 
ROSO.sub.3 Li, and mixtures of any two or all three of the foregoing 
wherein R is a hydrocarbon radical having 1 to 50 carbon atoms, in a 
solvent selected from the group consisting of N,N-dimethylacetamide, 
N,N-dimethylformamide, dimethylsulfoxide, and N-methyl-2-pyrrolidone, 
(b) storing the resulting solution in contact with stainless steel for a 
period of at least one day without the occurrence of substantial phase 
separation in the same or corrosive damage to the stainless steel, and 
(c) extruding the solution through an opening to form a shaped article. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
The polymeric material utilized in the present process is a linear 
polybenzimidazole. Typical polymers of this class and their preparation 
are more fully described in U.S. Pat. No. 2,895,948, U.S. Pat. No. Re. 
26,065, and in the Journal of Polymer Science, Vol. 50, pages 511-539 
(1961) which are herein incorporated by reference. The polybenzimidazoles 
consist essentially of recurring units of the following Formulas I and II. 
Formula I is: 
##STR1## 
wherein R is a tetravalent aromatic nucleus, preferably symmetrically 
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 class 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 class 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., from polymers 
consisting essentially of the recurring units of Formulas I and II wherein 
R' is an aromatic ring or a heterocyclic ring. 
As set forth in U.S. Pat. No. Re. 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-3,4-diaminobenzoate. 
As also 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 
orthodiamino substituents on the aromatic nucleus with a dicarboxyl 
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 carbon 
in a ring compound selected from the class 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'-cyclohexenyl-5,5'-bibenzimidazole; 
poly-2,2'-(m-phenylene)-5,5'-di(benzimidazole)ether; 
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)methane; 
poly-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. 
The preferred polybenzimidazole 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 in the present 
invention. Representative techniques for preparing the polybenzimidazole 
are disclosed in U.S. Pat. Nos. 3,509,108; 3,549,603; and 3,551,389, which 
are assigned to the assignee of the present invention and are herein 
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 200.degree. C., preferably at least 
250.degree. C., and more preferably from about 270.degree. to 300.degree. 
C. The reaction is conducted in a substantially oxygen-free atmosphere, 
i.e., below about 20 p.p.m. oxygen, until a foamed 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. 
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 hour, and preferably from about 1 to 4 hours or more. 
Suitable organic solvents for forming the stable non-corrosive solution of 
the present invention are those commonly used for dissolving a 
polybenzimidazole and are N,N-dimethylacetamide, N,N-dimethylformamide, 
dimethylsulfoxide, and N-methyl-2-pyrrolidone. The particularly preferred 
solvent is N,N-dimethylacetamide. 
The key to the present invention is the finding that a minor amount of an 
organo-lithium compound as defined hereafter when dissolved in the 
polybenzimidazole solution will inhibit the usual separation of the 
solution into phases of greater and lesser concentration of the 
polybenzimidazole upon the passage of time. This is accomplished without 
the promotion of corrosive stress cracking in stainless steel which 
contacts the same. The organo-lithium compound is selected from the group 
consisting of RCO.sub.2 Li, RSO.sub.3 Li, ROSO.sub.3 Li, and mixtures of 
any two or all three of the foregoing wherein R is a hydrocarbon radical 
having 1 to 50 carbon atoms. Representative lithium salts of 
monocarboxylic acids are lithium formates, lithium acetate, lithium 
propionate, lithium butyrate, lithium isobutyrate, lithium valerate, 
lithium isovalerate, lithium caproate, lithium laurate, 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 particularly preferred organo-lithium compound is 
lithium stearate. The organo-lithium compound may be dissolved in the 
solvent in a concentration of approximately 0.5 to 10 percent by weight 
based upon the weight of the solvent, and preferably in a concentration of 
approximately 2 to 5 percent (e.g. 2 percent) by weight based upon the 
weight of the solvent. In a preferred embodiment the organo-lithium 
compound is dissolved in the solvent prior to the dissolution of the 
polybenzimidazole in the same. 
The polybenzimidazole commonly is dissolved in the solvent in a 
concentration of approximately 10 to 30 percent by weight based upon the 
total weight of the solution, and preferably in a concentration of 
approximately 20 to 26 percent by weight when used in the formation of 
fibers. 
A preferred fiber spinning solution employs N,N-dimethylacetamide as 
solvent and comprises approximately 24 percent by weight of 
poly-2,2'-m-phenylene-5,5'-bibenzimidazole based upon the total weight of 
the solution, and approximately 2 percent by weight of lithium stearate 
based upon the weight of N,N-dimethylacetamide. 
The spinning solution preferably exhibits a viscosity of about 40 to 4000 
poises measured at 30.degree. C., and most preferably a viscosity of about 
1200 to 2500 poises measured at 30.degree. C. 
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. 
The polybenzimidazole solutions in accordance with the present invention 
contain no additives such as lithium chloride which may have an adverse 
influence on stainless steel following prolonged contact. Additionally, no 
phase out problem is encountered should the solution be stored following 
its formation and prior to extrusion to form a shaped article. For 
instance, the solution may be stored at least one day without phase 
separation, and preferably at least three days or more without phase 
separation. As indicated in the Examples, the solution may reliably remain 
stable many months in some embodiments. 
Shaped articles (e.g., fibers or films) may be formed from the 
polybenzimidazole solution of the present invention via standard extrusion 
techniques which are known in the art. For instance, the solution may pass 
through an orifice and into an evaporative or coagulative medium to form a 
shaped article. Representative techniques for forming shaped articles from 
the solution are disclosed in commonly assigned U.S. Pat. Nos. 3,441,640; 
3,502,756; 3,526,693; 3,584,104; and 3,619,453; and U.S. Pat. No. 32,422, 
filed Apr. 23, 1979 (now U.S. Pat. No. 4,263,245, granted Apr. 21, 1981). 
Such shaped articles may be washed and drawn in accordance with the 
teachings of commonly assigned U.S. Pat. Nos. 3,541,199; 3,622,660; 
3,657,411; 3,723,592; 3,743,479; 3,814,794; 3,816,581; 3,836,621; and 
3,848,529. Each of the above disclosures is herein incorporated by 
reference. Alternatively, films or membranes may be cast from the solution 
of the present invention. 
It has been found that filaments and membranes formed from the 
polybenzimidazole solution of the present invention following drawing and 
washing commonly possess a distinctive microporous internal structure. 
Such structure does not, however, result in any substantial reduction in 
tensile properties. Additionally, the resulting filaments commonly possess 
a delustered appearance and a substantially increased surface area (e.g. a 
four fold increase in surface area). It is expected that these surface 
characteristics may better facilitate dyeing of the same under appropriate 
conditions.

The following examples are presented as specific illustrations of the 
claimed invention. It should be understood, however, that the invention is 
not limited to the specific details set forth in the examples. 
EXAMPLE I 
The polybenzimidazole selected was 2,2'-m-phenylene-5,5'-bibenzimidazole 
having an inherent viscosity of approximately 0.75 (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.). 
A solution of lithium stearate and the 
2,2'-m-phenylene-5,5'-bibenzimidazole was formed in a one gallon stainless 
steel lined Parr reactor. More specifically, 11.4 grams of lithium 
stearate were dissolved in 570.0 grams of N,N-dimethylacetamide solvent 
when added to the solvent with agitation. The lithium stearate was 
commercially available from Phaltz and Bauer and was believed to be 100 
percent pure. The lithium stearate was accordingly present in the 
resulting solution in a concentration of 2 percent by weight based upon 
the weight of the N,N-dimethylacetamide solvent. 180.0 grams of the 
2,2'-m-phenylene-5,5'-bibenzimidazole were next dissolved in the solution 
of N,N-dimethylacetamide and lithium stearate by adding the polymer, 
purging the sealed vessel with nitrogen, and heating to approximately 
220.degree. C. The 2,2'-m-phenylene-5,5'-bibenzimidazole was accordingly 
present in the solution in a concentration of approximately 23.6 percent 
by weight based upon the total weight of the solution. No stress cracking 
of the stainless steel lining of the Parr reactor was observed. 
Samples of the resulting solution were placed in test tubes. One sample was 
maintained at room temperature (i.e. approximately 20.degree. C.) and the 
other sample was maintained at 100.degree. C. The samples were 
periodically observed for the possible presence of polarized particles 
using optical microscopy. The observation of any polarized particles was 
considered to be the onset of the phase out of the polymer. It was found 
that approximately twenty-two weeks elapsed before any polarized particles 
were observed in the solutions of the present invention. 
On the contrary control solutions which lacked the lithium stearate 
additive exhibited polarized particles much sooner. More specifically, the 
room temperature sample (i.e. approximately 20.degree. C.) exhibited 
polarized particles after 1 week and the 100.degree. C. sample after two 
weeks. 
EXAMPLE II 
Example I is substantially repeated with the exception that less pure 
lithium stearate from a different source was utilized. This sample was 
available from Eastman and was approximately 75 percent pure. 22.8 grams 
of the impure lithium stearate were dissolved in 570.0 grams of 
N,N-dimethylacetamide. Accordingly, the lithium stearate was present in 
the resulting solution in a concentration of approximately 3 percent by 
weight. The 2,2'-m-phenylene-5,5'-bibenzimidazole was provided in the same 
concentration as Example I. 
Substantially identical results were achieved as in Example I. 
EXAMPLE III 
Example I is substantially repeated with the exception that lithium acetate 
is substituted for the lithium stearate. 
EXAMPLE IV 
Example I is substantially repeated with the exception that lithium lauryl 
sulfonate is substituted for the lithium stearate. 
EXAMPLE V 
Example I is substantially repeated with the exception that lithium cetyl 
sulfate is substituted for the lithium stearate. 
Although the invention has been described with preferred embodiments, it is 
to be understood that variations and modifications may be employed as will 
be apparent to those skilled in the art. Such variations and modifications 
are to be considered within the scope of the claims appended hereto.