Wet Spinning Process For The Production Of Polybenzimidazole Filaments

Abstract

An improved wet spinning process is provided which is capable of producing highly uniform polybenzimidazole filaments which are substantially free of internal voids and radial incursions. A solution of the polybenzimidazole is extruded into a coagulation bath of certain polyhydroxy aliphatic alcohols to form an as-spun filament which is at least initially washed in a relatively cool wash medium, followed by drying, and stretching. The uniform filaments formed in the process are useful as precursors in the formation of carbon or graphite fibrous materials or as a substrate for metal deposition. When heavy denier filaments are formed in in the present process, these are particularly suited for use in the formation of fibrous closures.

Description

BACKGROUND OF THE INVENTION

Polybenzimidazoles are a known class of heterocyclic polymers which are characterized by a high degree of thermal stability. These polymers are described, for instance, in U.S. Pat. No. 2,895,948 to Brinker et al and in U.S. Reissue Pat. No. 26,065 to Marvel et al. Polybenzimidazole fibers are recognized to exhibit great resistance to degradation by heat, hydrolytic media, and oxidizing media.

Polybenzimidazole fibers of relatively low denier (e.g. from about 1 to about 10 denier per filament) have commonly been formed by various dry spinning techniques in which the spinning solution is extruded into a suitable evaporative medium to form fibers exhibiting a dogbone cross section. Polybenzimidazole fibers of essentially round cross sections are very rarely achieved through dry spinning processes. It also has not been possible to produce acceptable high denier (e.g. from about 50 to 2000 denier per filament) polybenzimidazole filaments by dry spinning techniques.

A process for the wet spinning of polybenzimidazole shaped articles in which a spinning solution of the polymer in sulfuric acid is extruded into a coagulation bath containing sulfuric acid at a concentration lower than that of the spinning solution is disclosed in U.S. Pat. No. 3,441,640 to Santangelo. Attempts to prepare an acceptable high denier polybenzimidazole fiber by this method failed because the filaments were too cracked and brittle.

Commonly assigned U.S. Pat. No. 3,526,693 of Richard N. Rulison and John P. Riggs discloses a wet spinning process for the formation of high denier polybenzimidazole filaments in which the resulting round filament is subjected to a series of controlled thermal healing treatments to heal radial incursions and to increase tensile properties.

It is an object of the invention to provide an improved wet spinning process for the formation of uniform as-spun polybenzimidazole filaments without the need to practice a thermal healing treatment to close voids and spin bath incursions.

It is an object of the invention to provide an efficient wet spinning process for the formation of polybenzimidazole filaments which possess smooth surface characteristics and are substantially free of internal voids and radial incursions.

It is an object of the invention to provide an efficient wet spinning process which in at least some of its embodiments is capable of forming polybenzimidazole filaments which are substantially round as well as substantially free of internal voids and radial incursions.

It is an object of the invention to provide an improved process which is particularly suited for the formation of uniform heavy denier polybenzimidazole filaments which are suitable for use in the manufacture of fibrous closures.

It is another object of the invention to provide essentially homogenous wet spun polybenzimidazole filaments which are suitable for use as precursors in the production of amorphous carbon or graphite fibers.

It is a further object of the invention to provide essentially homogenous wet spun polybenzimidazole filaments which are suitable for use as substrates for metal deposition.

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 an improved wet spinning process for the production of uniform polybenzimidazole filaments comprises:

a. providing a spinning solution of a fiber-forming polybenzimidazole dissolved in a solvent selected from the group consisting of dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, and N-methyl-2 -pyrrolidone,

b. extruding said spinning solution into a coagulation bath having a temperature of about 30.degree. to 100 .degree. C. selected from the group consisting of a polyhydroxy aliphatic alcohol having 2 to 3 hydroxy groups and 2 to 6 carbon atoms, mixtures of the polyhydroxy aliphatic alcohols, at least one of the polyhydroxy aliphatic alcohols in admixture with up to about 70 percent water by weight based upon the total weight of the mixture, and at least one of the polyhydroxy aliphatic alcohols in admixture with up to about 50 percent by weight of at least one of the solvents based upon the total weight of the mixture,

c. washing the resulting as-spun filament in water until substantially all residual amounts of the solvent and the coagulation bath are removed from the filament with the washing being initially conducted for at least 25 seconds with water at a temperature of about 5.degree. to 25.degree. C.,

d. drying the washed filament, and

e. heating the dried filament at a temperature of about 450.degree. to 600.degree. C. while simultaneously stretching the filament about 1.5 to 10 times its original length.

The resulting stretched filament may optionally be relaxed at a temperature of about 350.degree. to 525.degree. C.

DETAILED DESCRIPTION OF THE INVENTION

The Starting Polymer

The polybenzimidazoles which are wet spun according to the present process are the fiber-forming (fiber-formable) linear polybenzimidazoles. Typical polymers of this class and their preparation are more fully described in U.S. Pat. No. 2,895,948, U.S. Reissue Pat. No. 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: ##SPC1##

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: ##SPC2##

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 polybenzimidazole filaments are wet spun in the present invention, that is, filaments are prepared 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. Reissue 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- 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 ortho diamino 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'-ocatmethylene-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) 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: ##SPC3##

Any polymerization process known to those skilled in the art may be employed to prepare the polybenzimidazole which is wet spun to form a uniform filament in accordance with the process of the present invention. 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 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 hours 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 0.6, e.g. 0.80 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 hours, and preferably from about 1 to 4 hours or more.

THE SPINNING SOLUTION

The spinning solution of the polybenzimidazole selected for use in the present wet spinning process may be selected from those solutions commonly used for the dry spinning of polybenzimidazole filaments in accordance with the prior art. Solvents for the polybenzimidazole suitable for use in the formation of the spinning solution are dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, and N-methyl-2-pyrrolidone. The particularly preferred solvent is dimethylacetamide.

The polymer solutions may be prepared, for example, by dissolving sufficient polybenzimidazole in the solvent to yield a final solution suitable for extrusion containing from about 20 to 30 percent by weight of polymer, based on the total weight of the solution, and preferably from about 20 to 24 per cent by weight. The viscosity of the spinning solution should be within the range of 500 to 8000 poise, and preferably in the range of about 1500 to 4000 poise 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 are then filtered to remove any undissolved polymer which would otherwise clog the spinning orifice. A minor amount of lithium chloride optionally may be provided in the spinning solution in accordance with the teachings of commonly assigned U.S. Pat. No. 3,502,606 of Anthony B. Conciatori and Charles L. Smart.

WET SPINNING

The spinning solution of fiber-forming polybenzimidazole is extruded into a coagulation bath to form a filament having a smooth surface which is substantially free of internal voids and radial incursions. The homogeneous nature of the resulting as-spun filament is preserved during the subsequent steps of the process which further improve its usefulness.

It has been found that coagulation baths containing a substantial quantity of polyhydroxy aliphatic alcohol having 2 to 3 hydroxy groups and 2 to 6 carbon atoms are capable of producing highly uniform filaments in the wet spinning process of the invention. When used in combination with the subsequent steps of the process the need for a thermal healing treatment to heal radial incursions and to increase tensile properties as disclosed in U.S. Pat. No. 3,526,693 is totally obviated.

Representative polyhydroxy aliphatic alcohols of use in the present process include glycols such as ethylene glycol [1,2 -ethanediol], propylene glycol [1,2 -propanediol], trimethylene glycol [1,3 -propanediol], alpha-butylene glycol [1,2 -butanediol], beta-butylene glycol [1,3 -butanediol], tetramethylene glycol [1,4 -butanediol] , sym-dimethylethylene glycol [2,3 -butanediol] , diethylene glycol [2,2' -oxydiethanol] , triethylene glycol [2,2' -(ethylenedioxy)diethanol] , and hexamethylene glycol [1,6 -hexanediol]. Other polyhydroxy aliphatic alcohols such as glycerol [1,2,3 -propanetriol] may likewise be selected. The particularly preferred polyhydroxy aliphatic alcohol is ethylene glycol. Filaments formed in the present process utilizing ethylene glycol as the coagulation bath tend to be of a highly uniform round configuration.

The coagulation bath may contain a single polyhydroxy aliphatic alcohol, as defined, or a mixture of such alcohols. In another embodiment of the process at least one of the polyhydroxy aliphatic alcohols is present in admixture with up to about 70 per cent water by weight based upon the total weight of the mixture, and preferably not in excess of about 40 per cent water by weight, to form the coagulation bath. Such coagulation baths which include water have a tendency to increase the rate of coagulation. Extremely rapid coagulation resulting from the use of a greater proportion of water tends to result in the introduction of voids and incursions into the resulting filaments. Alternatively, the coagulation bath may contain a mixture of at least one of said polyhydroxy alcohols with up to about 50 per cent by weight based upon the total weight of the mixture of at least one of the spinning solution solvents, and preferably not in excess of about 20 percent of at least one of the solvents. Such coagulation baths containing an appreciable quantity of the solvent have a tendency to decrease the rate of coagulation. If more than about 65 per cent by weight of at least one of the solvents is present in the coagulation bath, this results in little or no fiber formation at practical spinning speeds depending upon the specific coagulation bath.

The temperature of the spinning solution during the extrusion should be within the range of 10.degree. to 180.degree. C. and preferably at about 25.degree. to 80.degree. C. The spinneret should contain holes of a diameter between about 40 to 150 microns when producing relatively low denier filaments, and holes of about 200 to 600 microns when producing relatively high denier filaments. Monofilaments or continuous multifilament yarns may be formed. The extrusion pressure should be between 100 and 750 p.s.i. Spinning or extrusion speeds of about 5 to 50 meters per minute, and preferably about 10 to 20 meters per minute may be selected.

The coagulation bath containing at least one polyhydroxy aliphatic alcohol into which the spinning solution is extruded is maintained at a temperature of about 30.degree. to 100.degree. C., and preferably at a temperature of about 30.degree. to 60.degree. C. If the coagulation bath is maintained at a temperature much above 100.degree. C., then voids and incursions may result. If the coagulation bath is maintained at a temperature much below 30.degree. C., then insufficient coagulation may result. The exact temperature selected may be influenced by the extrusion speed, the length of the coagulation bath, and the composition of the coagulation bath.

The resulting as-spun filament is commonly present in the coagulation bath for a residence time of about 1 to 15 seconds, and preferably for a residence time of about 2 to 6 seconds.

WASHING AS-SPUN FILAMENT

The resulting as-spun filament is next washed with water until substantially all residual amounts of solvent and coagulation bath are removed from the same. It is essential that the filament first be exposed to a relatively cool water wash medium at about 5.degree. to 25.degree. C., and preferably at a temperature of at least about 10.degree. to 15.degree. C., for at least about 25 seconds. The entire wash treatment may be conducted at such relatively cool temperature, or the water wash medium may be subsequently raised to a more highly elevated temperature. The initial cool water wash has been found to be essential in order to preserve filament homogeneity.

When the entire wash is conducted at a relatively cool wash temperature of about 5.degree. to 25.degree. C., wash times of about 24 to 100 hours are commonly required depending upon the filament denier.

During the cool water wash a one-way transfer of residual quantities of the spinning solvent out of the filament is believed to be promoted to the substantial exclusion of the passage of the molecules of the wash medium into the filament. Such wash treatment has been found to be highly important if the homogeneous nature of the filament is to be preserved. For instance, it has been found that if the as-spun filament is initially washed at a temperature substantially higher than about 25.degree. C. then the resulting washed fiber tends to lack homogeneity.

In a preferred embodiment of the invention, an initial cool water wash step is followed by a subsequent wash step at a more highly elevated temperature. For instance, the filament may be initially water washed at a temperature of about 5.degree. to 25.degree. C. for about 25 seconds to 5 minutes or more, and subsequently at a temperature of about 30.degree. to 100.degree. C., preferably at a temperature of about 50 to 70.degree. C., for about 16 to 50 hours or more. Longer residence times may be tolerated without deleterious results, but tend to yield no commensurate advantage.

During this subsequent wash step the filament is rendered substantially free of the spinning solution and coagulation bath. It has been found that the subsequent hot water wash may be tolerated by the filament when preceded by the initial cool wash step. Exhaustive water washing has been found essential in order to permit effective drawing and to prevent destructive volatilization of residual solvent and coagulant. The residual solvent content of the washed filament should contain no more than about 0.1 percent solvent by weight and preferably no more than about 0.05 percent solvent by weight.

The initial cold water wash is conveniently conducted in an in line operation with the filament after it leaves the coagulation bath continuously passing through a wash medium which is continuously regenerated. Conventional filament wash rolls may be utilized. The filament additionally may be washed while wound upon a perforated bobbin or by the use of any other convenient means as will be apparent to those skilled in the art.

DRYING OF WASHED FILAMENT

The washed filament is translucent and is next dried by any convenient means to form a filament which is essentially free of moisture. For instance, the filament may be continuously passed over at least one steam heated roll, or the filament while present on a bobbin may be placed in an oven. Drying may be conveniently conducted in stages carried out at successively elevated temperatures. For instance, drying may be conducted for about 24 hours at room temperature, 16 to 24 hours at 110.degree. to 130.degree. C., and for 16 to 100 hours at 160.degree. C. Gradual drying of the washed filament tends to minimize coalescence. In order to avoid difficulties created by moisture regain from the atmosphere during the subsequent drawing step, the dried filament may be placed under an infrared lamp prior to drawing. For instance, an infrared lamp may be simply projected onto the feed bobbin for the drawing zone.

DRAWING OF DRY FILAMENT

The essentially dry filament is next heated at a temperature of about 450.degree. to 600.degree. C. while simultaneously stretched about 1.5 to 10 times its original length. The dense uniform filament microstructure makes possible such relatively high draw ratios. In a preferred embodiment of the invention the filament is drawn about 2.5 to 3.5 times its original length while heated at a temperature of about 475.degree. to 500.degree. C. Temperature profiling within the draw chamber is recommended in which the filament is gradually elevated to the draw temperature indicated over a period of at least one second or more.

The drawing of the filament may be conducted in any convenient manner such as by applying tension while passing the filament one or more times over a hot shoe or heated roll, or through a muffle furnace. The drawing step of the process improves tensile properties of the filament. Preferred drawing techniques are disclosed in U.S. Ser. No. 769,862, filed Oct. 23, 1968 in the names of George F. Ecker and Thomas C. Bohrer, and U.S. Pat. No. 3,541,199 in the names of Thomas C. Bohrer and Arnold J. Rosenthal. These are assigned to the same assignee as the present invention and are herein incorporated by reference.

RELAXATION OF DRAWN FILAMENT

The resulting drawn filament optionally next may be relaxed. For such treatment the drawn filament is heated to a temperature of about 350.degree. to 525.degree. C., and is allowed to achieve a relaxation of about 10 to 65 percent in length, and preferably about 30 to 50 percent in length. A preferred relaxation temperature is about 425.degree. to 475 C. The relaxation step of the process renders the filament more highly extensible and greatly improves the knot strength of the same.

The relaxation step may be conveniently conducted while passing the drawn filament through a muffle furnace one or more passes or by passing the same over a hot shoe, or shoes. Other relaxation techniques will be apparent to those skilled in the art.

High denier polybenzimidazole monofilaments formed in accordance with the present invention having a denier of a least 100, and preferably at least 200, and a uniform round cross section find utility in the manufacture of fibrous closures. For instance, one member of the closure may contain pile or loops and the other member inter engageable stiff, resilient hooks generally made by cutting looped filaments. Separation requires a force of a considerable magnitude when release of a large number of hooks at once is attempted, but separation may be quite readily effected by progressively peeling the layers apart. In specialized uses, these fibrous closures are preferable to conventional closures, such as metal zippers since they (1) can be quickly opened, (2) can be quickly closed, i.e. no special orientation is required, and (3) cannot get "stuck" or easily break. Air and sea craft represent an important area where such advantages are highly desirable. Fibrous closures have heretofore been used in such environments as a means of quick, easy and reliable attachment and detachment. For example, fibrous closures are widely used in the interior of commercial aircraft to permit easy and nondestructive stripping for periodic examination of the aircraft frame and structure. The polybenzimidazole filaments of the present process are invaluable in the production of fibrous closures for use in space and undersea vehicles where materials which are resistant to flames and high temperatures are required.

A fibrous closure containing the high denier polybenzimidazole monofilaments formed in accordance with the present process can be fabricated in a known manner, as for example that disclosed by De Mestral in U.S. Pat. Nos. 2,717,437; 3,009,235; 3,154,837 and 3,136,026. For example, they can be woven into the supporting material in the conventional manner of making a pile fabric and then cut. In still another method hooking members can be applied to a plastic support by electrostatic dispersion and gluing. In a preferred embodiment one member has all the hooks and they are randomly arranged while the other member has all the loops and they are oriented. It is of course possible for each member to have hooks and loops. A small loop can be tied in the monofilament and contacted with a hot metal surface, e.g. a hot shoe at about 350.degree. to 550.degree. C. for several seconds. Upon cooling and being untied, the loops will have a permanently set configuration in the filament and can be repeatedly straightened under tension without apparent loss of resiliency. This heat-setting operation is preferably effected in situ, i.e. while an integral part of the closure member. It may be effected before adhesion, however.

Aside from their use in the aforedescribed closures, the relatively heavy denier monofilaments can also advantageously replace other fibrous materials in such uses as high-speed, heat-generating machinery, firemen's clothing, and the like. In view of their stiffness and resiliency they may also be used as cleaning, scouring and/or brushing means such as in clothes brushes and shoe brushes.

Polybenzimidazole filaments formed in accordance with the present process of relatively low denier may likewise be utilized in the formation of heat resistant, nonburning fabrics.

The polybenzimidazole filaments of either high or low denier also find utility as highly uniform precursors for the production of carbon or graphite fibrous material upon further thermal treatment. Such carbon or graphite fibrous materials are particularly useful as a reinforcing medium when embedded in resinous matrix to form a strong lightweight structural component. The polybenzimidazole filaments may additionally be used as substrates for the deposition of metals such as boron to form similar reinforcing media.

The following examples are given as specific illustrations of the invention. It should be understood, however, that the invention is not limited to the specific details set forth in the examples.

EXAMPLE I

A spinning solution having a Brookfield low shear viscosity of 2100 poise at 30.degree. C. is prepared employing dimethylacetamide as solvent containing 22.5 percent by weight poly[2, 2' -(m-phenylene)-5, 5' -bibenzimidazole] based upon the total weight of the solution, and 2 per cent by weight lithium chloride based upon the total weight of the solution. A preparation of the polybenzimidazole is described in example II of U.S. Reissue Pat. No. 26,065.

The spinning solution while at a temperature of 25.degree. C. is fed to a spinneret having a single hole of 316 micron diameter and is extruded at a rate of 10 meters per minute into a circulating coagulation bath of ethylene glycol moving concurrently with the resulting filament having a temperature of 30.degree. C. The resulting as-spun filament remains in the coagulation bath for approximately 5 seconds.

The coagulated filament is passed to an in line wash roll and washed with circulating water at a temperature of 10.degree. C. for 120 seconds, and then passed to an in line wash roll and washed with circulating water at a temperature of 30.degree. C. for 170 seconds. Next the filament is pressure washed while present on a perforated bobbin with circulating water at about 15.degree. C. for 100 hours. The resulting translucent filament is free of residual quantities of the ethylene glycol coagulation bath and contains less than 0.05 percent by weight dimethylacetamide.

The filament while present on the perforated bobbin is dried for 20 hours at room temperature (i.e. 25.degree. C.), 16 hours at 115.degree. C., and 100 hours at 160.degree. C. The moisture content of the dried filament is less than about 0.05 percent by weight.

The dried filament is drawn at a draw ratio of 3.1:1 while passing through a Lindberg muffle furnace provided at a maximum temperature of 510.degree. C. at a supply speed of 20 meters per minute. The drawn filament is next relaxed (rolls at a ratio of 20:11) 45 percent of its length by passage for a residence time of 16 seconds through a muffle furnace at 475.degree. C. at a supply speed of 20 meters per minute.

The physical properties (straight tensiles) of the drawn and relaxed filament are:

Denier per filament 204 Elongation (%) 46.2 Tenacity (grams per denier) 3.58 Modulus (grams per denier) 69.7

The physical testing is conducted at 70.degree. F. and at 65 percent relative humidity. When submitted to microscopic examination, the filament exhibits a smooth uniform round cross section which is free of voids and radial incursions.

EXAMPLE II

A spinning solution having a Brookfield low shear viscosity of 1600 poise at 30.degree. C. is prepared employing dimethylacetamide as solvent containing 22.0 percent the polybenzimidazole employed in example I based upon the total weight of the solution, and 2 percent by weight lithium chloride based upon the total weight of the solution.

The spinning solution while at a temperature of 25.degree. C. is fed to a spinneret having a single hole of 250 micron diameter and is extruded at a rate of 20 meters per minute into a coagulation bath of ethylene glycol having a temperature of 50.degree. C. The resulting as-spun filament remains in the coagulation bath for 2.5 seconds.

The coagulated filament is passed to an in line wash roll and washed with circulating water at a temperature of 10.degree. C. for 60 seconds, and then passed to an in line wash roll and washed with circulating water at a temperature of 80.degree. C. for 85 seconds. Next the filament is pressure washed while present on a perforated bobbin with circulating water at about 60.degree. C. for 96 hours. The resulting translucent filament is free of residual quantities of the ethylene glycol coagulation bath and contains less than 0.05 percent by weight of dimethylacetamide.

The filament while present on a perforated bobbin is dried for 24 hours at room temperature (i.e. 25.degree. C.), 30 hours at about 105.degree. C., and 96 hours at 160.degree. C. The moisture content of the dried filament is less than about 0.05 percent by weight.

The dried filament is drawn at a draw ratio of 3.1:1 in a Lindberg muffle furnace provided at a maximum temperature of 485.degree. C. at a supply speed of 10 meters per minute. The drawn filament is next relaxed (rolls at ratio of 20:10 ) 50 percent of its length by passage for a residence time of 16 seconds through a muffle furnace at about 480.degree. C. at a supply speed of 20 meters per minute.

The physical properties of the drawn and relaxed filament having a denier per filament of 220 are as follows:

Sraight Tensiles Knot Tensiles __________________________________________________________________________ Elongation (%) 52 25 Tenacity (grams per denier) 3.15 2.28 Modulus (grams per denier) 68 -- __________________________________________________________________________

The physical testing is conducted at 70.degree. F. and at 65 percent relative humidity. When submitted to microscopic examination, the filament exhibits a smooth uniform round cross section which is free of voids and radial incursions.

EXAMPLE III

Example II is repeated with the exception that the coagulation bath is glycerol. Smooth uniform filaments are formed having a flat dogbone configuration which are free of voids and radial incursions.

EXAMPLE IV

Example II is repeated with the exception that the coagulation bath is glycerol in admixture with 40 percent water based upon the total weight of the mixture. Smooth uniform filaments are formed having a round to oval configuration which are free of voids and radial incursions.

EXAMPLE V

Example II is repeated with the exception that the coagulation bath is ethylene glycol in admixture with 20 percent dimethylacetamide based upon the total weight of the mixture. Smooth uniform round filaments are formed which are free of voids and radial incursions.

EXAMPLE VI

Example II is repeated with the exception that the spinneret is provided with 100 holes of 50 microns each. A continuous multifilament yarn is formed composed of uniform round filaments which are free of voids and radial incursions.

Although the invention has been described with preferred embodiments, it is to be understood that variations and modifications may be resorted to as will be apparent to those skilled in the art. Such variations and modifications are to be considered within the purview and scope of the claims appended hereto.

Claims

I claim:

1. An improved wet spinning process for the production of polybenzimidazole filaments comprising:

a. providing a spinning solution of a fiber-forming polybenzimidazole dissolved in a solvent selected from the group consisting of dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, and N-methyl-2-pyrrolidone,

b. extruding said spinning solution into a coagulation bath having a temperature of about 30.degree. to 100.degree. C. selected from the group consisting of a polyhydroxy hydrocarbon alcohol having 2 to 3 hydroxy groups and 2 to 6 carbon atoms, mixtures of said polyhydroxy hydrocarbon alcohols, at least one of said polyhydroxy hydrocarbon alcohols in admixture with up to about 70 percent water by weight based upon the total weight of the mixture, and a least one of said polyhydroxy hydrocarbon alcohols in admixture with up to about 50 per cent by weight of at least one of said solvents based upon the total weight of the mixture,

c. washing the resulting as-spun filament in water until substantially all residual amounts of said solvent and said coagulation bath are removed from said filament with said washing being initially conducted for at least 25 seconds with said water at a temperature of about 5.degree. to 25.degree. C.,

d. drying said washed filament, and

e. heating said dried filament at a temperature of about 450.degree. to 600.degree. C. while simultaneously stretching said filament about 1.5 to 10 times its original length.

2. An improved process according to claim 1 which includes the additional step of relaxing said stretched filament at a temperature of about 350.degree. to 525.degree. C.

3. An improved process according to claim 1 wherein said fiber-forming polybenzimidazole consists essentially of recurring units of the formula: ##SPC4##

wherein R is a tetravalent aromatic nucleus, with the nitrogen atoms forming the benzimidazole rings paired upon adjacent carbon atoms of said aromatic nucleus, and R' is selected from the group consisting of (1) an aromatic ring, (2) an alkylene group having from four to eight carbon atoms, and (3) a heterocyclic ring selected from the group consisting of (a) pyridine, (b) pyrazine, (c) furan, (d) quinoline, (e) thiophene, and (f) pyran.

4. An improved process according to claim 1 wherein said polybenzimidazole is poly 2,2' -(m-phenylene)-5,5' -bibenzimidazole.

5. An improved process according to claim 1 wherein said solvent utilized in the formation of said spinning solution is dimethylacetamide.

6. An improved process according to claim 1 wherein said spinning solution contains 20 to 30 percent by weight based upon the total weight of the solution of poly 2,2'-(m-phenylene)-5,5'-bibenzimidazole dissolved in dimethylacetamide.

7. An improved process according to claim 1 wherein said coagulation bath has a temperature of about 30.degree. to 60.degree. C.

8. An improved process according to claim 1 wherein said coagulation bath is ethylene glycol.

9. An improved process according to claim 1 wherein said coagulation bath is glycerol.

10. An improved process according to claim 5 wherein said coagulation bath is a mixture of ethylene glycol and dimethylacetamide containing up to about 20 percent by weight dimethylacetamide based upon the total weight of said mixture.

11. An improved process according to claim 1 wherein said coagulation bath is a mixture of glycerol and water containing up to about 40 percent by weight water based upon the total weight of said mixture.

12. An improved process according to claim 1 wherein said resulting as-spun filament is initially washed in water at a temperature of about 10.degree. to 15.degree. C.

13. An improved process for the production of a polybenzimidazole filament comprising:

a. providing a spinning solution of poly 2,2'-(m-phenylene)-5,5' -bibenzimidazole dissolved in dimethylacetamide in a concentration of about 20 to 24 percent by weight based upon the total weight of the solution,

b. extruding said spinning solution into an ethylene glycol coagulation bath having a temperature of about 30.degree. to 60.degree. C.,

c. initially washing the resulting as-spun filament for at least 25 seconds in water at a temperature of about 5.degree. to 25.degree. C.,

d. subsequently washing the filament in water at a temperature of about 30.degree. to 100.degree. C. until substantially all residual amounts of dimethylacetamide and ethylene glycol are removed from said filament,

e. drying said washed filament, and

f. heating said dried filament at a temperature of about 450.degree. to 600.degree. C. while simultaneously stretching said filament about 1.5 to 10 times its original length.

14. An improved process according to claim 13 which includes the additional step of relaxing said stretched filament about 10 to 65 percent in length at a temperature of about 350.degree. to 525.degree. C.

15. An improved process according to claim 13 in which said step(e) is conducted at about 475.degree. to 500.degree. C. while simultaneously stretching said filament about 2.5 to 3.5 times its original length.