United States Patent Office 3,541,199 Patented Nov. 17, 1970 US. Cl. 264-290 13 Claims ABSTRACT OF THE DISCLOSURE Foamable polybenzimidazole fiber or yarn (strand material) which undergoes foaming or exploding during hot drawing is subjected, prior to the hot drawing step, to a non-degrading heat treatment at a temperature in the range of from about 50 C. below the hot drawing temperature up to the degradation temperature of the polybenzimidazole fiber or yarn for a period of time of at least about 1 second.
RELATED APPLICATIONS This application is a continuation-in-part of application Ser. No. 520,657, filed Jan. 14, 1966 now abandoned, which application is assigned to the assignee of the present invention.
BACKGROUND OF THE INVENTION The present invention relates to a process for improving the tensile properties such as tenacity of foamable polybenzimidazole fibers, filaments, yarns and the like, sometimes hereinafter referred to generally as strand material. More particularly, the present invention relates to a process for heat treating foamable polybenzimidazole fiber prior to the fiber being hot drawn so as to eliminate or overcome the foaming or exploding problem which frequently occurs thereby yielding a polybenzimidazole fiber having improved tensile properties.
Hot drawing or stretching polymer fibers is a well known method for increasing the fiber tensile properties, particularly tenacity. It has been found, however, that when foamable polybenzimidazole fiber, as hereinafter defined, and particularly foamable aromatic polybenzimidazole fiber is subjected to a hot drawing operation, the fiber will foam or explode during the operation resulting in a porous fiber product having relatively low tensile properties.
Heretofore, it Was believed that the foaming problem was due in large part to the presence of water in the polybenzimidazole fiber, the water having been picked up primarily during the water washing step used to remove residual spinning solvent from the fiber. However, it has since been found that even when the polybenzimidazole fiber is dried to remove essentially all of the wash liquid such as water, the resulting fiber frequently still foams during the hot drawing operation. Thus, drying alone is not the solution. It appears that certain unknown volatile materials that may be formed during the polybenzimidazole polymerization reaction and which tenaciously adhere or are chemically bound to the fibers may be chiefly responsible for the foaming problem during hot drawing.
Several methods have been suggested for overcoming the foaming problem. For example, the deleterious foaming and exploding can be reduced by hot drawing at low speeds, that is, the speed of the supply roll, on the order of 5 meters a minute. However, such drawing operations are too slow to permit large scale economic production.
Similarly, the foaming and exploding can be reduced by hot drawing at low draw ratios on the order of about 1.5 :1 or less, for example, as determined by the ratio of the surface speed of the takeup or stretch roll to the surface speed of the supply roll. However, such low draw ratios are undesirable because a high degree of orientation cannot thereby be achieved, and fibers of low orientation have low tenacity values.
Accordingly, the primar object of the present invention is to provide a process for improving the tensile properties of foamable polybenzimidazole fiber or yarn, that is, strand material. Another object is to provide a process for hot drawing foamable polybenzimidazole fiber without foaming. These and other objects will be apparent from the following description,
In accordance with the present invention, foamable polybenzimidazole fiber or yarn, i.e., strand material, is subjected to a heat treatment prior to the hot drawing operation. The heat treatment is conducted so as not to degrade the polybenzimidazole strand material, that is, a non-degrading heat treatment, at a temperature in the range of from about 50 C. below the hot drawing temperature up to the degradation temperature of the polybenzimidazole material for a period of time of at least about one second. I
The term foamable polybenzimidazole fiber or yarn is meant to include any and all polybenzimidazole fiber or yarn (strand material) which undergoes foaming or exploding or both during hot drawing, that is, drawing at a temperature above about 400 C., particularly when the hot drawing is at a draw ratio of above about 1.5 :1, and at a drawing speed in excess of about 5 to 10 meters per minute.
It is important to note that not all polybenzimidazole strand material undergoes foaming during hot drawing, irrespective of the drawing speeds and ratios. However, there is no present method for determining whether or not foaming will occur without actually hot drawing the material in a simple test run. Hence, if foaming does occur, the hot drawing process of the present invention may be employed.
DETAILED DESCRIPTION OF THE INVENTION The starting polymer Polybenzimidazoles are a known class of heterocyclic polymers which consist essentially of recurring units of the following Formulas I and II. Formula I is:
wherein R is a tetravalent aromatic nucleus, 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 an aliphatic (alkylene) group, a cycloaliphatic ring, an aromatic ring and a heterocyclic ring such as pyridine, pyrazine, furan, quinoline, thiophene, and pyran. Formula II is:
sisting essentially of the recurring units of Formula II and of Formula I wherein R is an aromatic ring or a heterocyclic ring.
As set forth in US. Pat. No. 3,174,947 and Re. Pat. 26,065, which are incorporated herein by reference, the aromatic polybenzimadazoles having the recurring units of Formula Il 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 auto-condensation of phenyl-3,4-diaminobenzoate.
As also set forth in the above-mentioned patents, 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) and anhydride of a aromatic dicarboxylic acid.
Examples of aromatic polybenzimidazoles which have the recurring structure of Formula I and which may be formed into fibers or yarns include:
where the double bonds of the ethylene groups are intact in the final polymer.
The preferred aromatic polybenzimidazole fiber or yarn is one prepared from poly-2,2-(m-phenylene)-5,5'-bibenzimidazole, the recurring unit of which is:
Any polymerization process known to those skilled in the art may be employed to prepare the polybenzimidazole which may then be spun into fiber and if it foams during hot drawing, subjected to the heat treatment of the present invention.
With respect to aromatic polybenzimidazoles, preferably equimolar quantities of the monomeric tetraamine and dicarboxyl compound are introduced into a first stage melt polymerization reaction zone and heated therein at a temperature above about 200 C., preferably at least 250 C., and more preferably from about 270 to 300 C. The reaction is conducted in a substantially oxygen-free atmosphere, i.e., below about 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, the inherent viscosity (I.V.) as used in the present specification and claims being determined from a solution of 0.4 gram of the polymer in 100 ml. of 97 percent H 80 at 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 pulzerized 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 0, preferably at least 325 C., and more preferably from about 350 to 425 C. The second stage reaction generally takes at least 0.5 hour, and preferably from about 1 to 4 hours or more.
Preparation of the fiber As is well known, the polybenzimidazoles are generally formed into fibers or filaments by solution spinning, that is, by dry or wet spinning a solution of the polymer in an appropriate solvent such as dimethylacetamide, dimethylformamide, dimethylsulfoxide or sulfuric acid (used only in wet spinning) through an opening of predetermined shape into an evaporative atmosphere for the solvent in which most of it is evaporated (dry) or into a coagulation bath (wet), resulting in the polymer having the desired shape.
The polymer solutions may be prepared, for example, by dissolving sufficient polybenzimidazoles in the solvent to yield a final solution suitable for extrusion containing from about 10 to 45% by weight of polymer, based on the total weight of the solution, preferably from about 20 to 30% by weight.
One suitable means for dissolving the polymer in the solvent is by mixing the materials at a temperature above the atmospheric boiling point of the solvent, for example, 25 to C. above such boiling point, and at a pressure of 2 to 15 atmospheres for a period of 1 to 5 hours.
Preferably, the above polymer solutions, after suitable filtration to remove any undissolved portions, are dry spun. For example, the solutions may be extruded through a spinneret into a conventional type downdraft spinning column containing a circulating inert gas such as nitrogen, noble gases, combustion gases or super heated steam. Conveniently, the spinneret face is at a temperature of from about to 170 C., the top of the column from about 120 to 220 C., the middle of the column from about to 250 C., and the bottom of the column from about to 320 C. After leaving the spinning column, the fibers are taken up, for example, at a speed in the range of from about 50 to 350 meters per minute. The resulting as-spun fibers are then preferably subjected to a slight steam drawing treatment at a draw ratio of from about 1.1:1 to 1.5 :1. This pre-wash drawing treatment is to be distinguished from the post-wash hot drawing process of the present invention. The pre-wash drawing is employed in order to prevent the fibers from relaxing and falling off the bobbin during the subsequent washing step.
Desirably, the fibers are next washed so as to remove residual spinning solvent, e.g., so that the washed fiber contains less than about 1% by weight solvent, based on the weight of the fiber, and preferably so as to obtain an essentially spinning solvent-free fiber. Typically, a simple water wash is employed, however, if desired other wash materials such as acetone, methanol, methyl ethyl ketone and similar dimethylacetamide-miscible and volatile organic solvents may be used in place of or in combination with the water. The washing operation may be conducted by collecting the polybenzimidazole fiber on perforated rolls or bobbins, immersing the rolls in the liquid wash bath and pressure washing the fiber, for example, for about 2 to 48 hours or more.
The washed fibers are then dried in any suitable type of apparatus such as an electric oven to remove at least the major portion of the wash liquid, for example, at least 60% by weight of the wash liquid, preferably at least 90% by weight, and more preferably essentially all of the wash liquid. If substantially less than about 60% of the wash liquid is removed prior to subjecting the fiber to hot drawing the fiber may undergo foaming merely due to the excessive wash liquid content. As previously mentioned, however, even if essentially all of the wash liquid is removed, this alone will not overcome the foaming problem which occurs during the hot drawing operation, clearly indicating that the solution to the foaming is not merely drying.
The drying operation is conveniently conducted at a temperature of from about 150 to 300 C. for about 2 to 100 hours or more. Preferably, the drying temperature does not exceed about 250 C. as above this temperature degradation of the fiber may occur, particularly if an extended heating period, i.e., several hours, is employed. Degradation is the loss of physical properties of a polymer fiber due to severing of the polymer chains due to heat or chemical reactions or both.
Heat treatment As previously mentioned, certain polybenzimidazole strand material undergoes foaming or exploding if it is subjected to hot drawing subsequent to the drying operation. The essence of the present invention is the elimination of this problem by the use of a particular heat treating step or operation.
The heat treatment is conducted by heating the foam able fiber or yarn at a temperature no lower than about 50 C. below the temperature employed in the hot drawing operation, preferably from about 50 C. below the hot draw temperature, that is, the maximum temperature used during the hot drawing operation, up to about the degradation temperature of the polybenzimidazole fiber. The .so-called degradation temperature varies for each polybenzimidazole, but may be determined by sim- 'ple experimentation. While the heat treatment temperature can be higher than the hot draw temperature up to the degradation temperature, most preferably the heat treatment temperature is essentially the same as the hot draw temperature, i.e., maximum temperature employed in the hot drawing operation.
The heat treatment is conducted for at least about one second, as below this time period the exposure is insufiicient to achieve the desired results of overcoming the hot drawing foaming problem. Preferably, the residence time for the heat treatment is from about 5 to 50 seconds and, more preferably, from about 5 to 30 seconds. Typically, the higher the temperature the lower the residence time that is necessary to achieve the non-foaming fiber structure. Above about 50 seconds residence time, the fiber will usually undergo degradation, particularly at the higher temperatures, for example, above the hot draw temperature.
The heat treatment may be conducted in any suitable type apparatus or zone either on a batch, continuous or semi-continuous basis, provided that the individual fiber, filament, or yarn is in a substantially exposed condition. For example, it has been found that the strand material cannot merely be wound in a typical overlapping fashion onto a bobbin, placed into a furnace and heated. While the fiber near the top, that is furthest from the bobbin surface, will have been sufficiently treated so as not to foam during hot drawing, the fiber underneath, near the bobbin will still undergo foaming.
The preferred method for heat treating the strand material is continuously, for example, by passing the fiber or filament over a hot surface such as a hot shoe, pin or plate or through a radiantly heated zone such as a muffie furnace. The strand material may also be continuously heated by helically Wrapping it (without substantial overlapping) around an internally heated rotating circular 7 surface. Employing continuous treatment assures adequate exposure of the material.
With respect to a suitable batch treatment, the fiber may be wrapped, without substantial overlapping, onto a spool or bobbin and placed into any conventional type heating zone such as an oven. Obviously, such a batch treatment is not practical on a commercial basis where speed and volume production are important factors.
The fiber or filament used in the present invention conveniently has a denier of from about 1 to 20 or more. Normally higher fiber deniers require slightly higher heat treatment residence times within the aforementioned ranges in order to achieve the desired non-foaming condition. The optimum or preferred residence time for each polybenzimidazole fiber and denier may be easily ascertained by simple experimentation.
It is important to note that the invention is equally applicable to all forms of foamable polybenzimidazole strand material, that is, fibers, filaments, yarns or threads and the like. With multi-filament materials, however, continuous heat treatment is even more preferred in order to achieve adequate exposure of the individual fibers or filaments.
Hot drawing As a result of the aforementioned heat treatment, the polybenzimidazole strand material may be hot drawn without any foaming occurring, thereby enabling non-porous polybenzimidazole material to be obtained. In addition, because of the heat treatment, high drawing speeds, that is, the speed of the supply roll, on the order of 20 to 50 meters per minute or higher may be used, the limiting factor being the particular design of the drawing apparatus and the elastic properties of the fiber or yarn, and not the foaming of the polybenzimidazole as heretofore. Further, the elimination of the foaming problem allows higher draw ratios to be used if so desired.
The resulting heat treated polybenzimidazole fiber or yarn is subjected to a hot drawing treatment in any convenient hot drawing apparatus or zone, for example, by passing the fiber over a heated surface such as a hot roll, shoe, pin or plate or by passing the fiber through a radiantly heated zone such as a muffle furnace.
' Preferably, the strand material is passed immediately from the heat treatment zone into the hot drawing zone in order to minimize exposure of the heat treated material to the atmosphere as polybenzimidazole fiber or yarn combines readily with the moisture in the air. The strand material may absorb as much as 18% by weight of water from the air.
The hot drawing is suitably conducted at a temperature of above about 350 C. up to about the degradation temperature of the polybenzimidazole fiber or yarn, preferably from about 450 to 650 C., and more preferably from about 500 to 550 C.
The pre-heated strand materials may be drawn at any desired draw ratio below that at which they break, typically from above about 1.5 :1 to 3.5 :l and preferably from about 2.011 to 3.2:1. The hot drawing residence time, i.e., the time during which the fiber or yarn is being heated, is usually from about 1 to 50 seconds, preferably 5 to 30 seconds, and more preferably 10 to 20 seconds. Substantially longer residence times at the elevated drawing temperatures may cause fiber degradation.
The resulting hot drawnpolybenzimidazole fibers or yarns, i.e., strand materials, of the present invention are characterized by a high degree of thermal stability and show great resistance to degradation by heat, hydrolytic media and oxidizing media. They may be used, for example, in deceleration chutes for aircraft, chutes for re-entry capsules, high temperature dust collector bags and nonfiammable clothing and fabrics.
The invention is additionally illustrated in connection with the following example.
7 EXAMPLE A foamable polybenzimidazole yarn, namely, poly-2,2- (m-phenylene)-5,5-bibenzimidazole, which may be prepared in any convenient manner known to those skilled in the art as heretofore described was used in each of the following runs. I
In each run in Table I below the yarn was drawn over a hot shoe at 425 C. The method of thermal pretreatment employed was to pass the yarn through a radiantly heated 8 on a continuous basis at essentially the same temperature as employed in the hot drawing operation for a period of time in the range of from about to 30 seconds, and thereafter subjecting the resulting heat treated material to a hot drawing operation at a temperature in the range of from about 450 to 650 C. for a period of time in the range of from about 5 to 30 seconds and a draw ratio from above about 1.5:1 to 3.5:1 to yield a polybenzimidazole strand material having improved tensile properties.
mutfie furnace. 0 5. The process of claim 4 wherein a draw ratio in the TABLE I Draw speed (meters/min.) I
Elongation, Tenacity,
Thermal pretreatment R R2 Draw ratio percent grams/denier 'IE /z (1) NOne Control-N0 Pretreatment or drawing 116 1. 9 30 (2) 12 seconds, 482 C 50 60 1. 2:1 65 3. El 31 (3) None 2.011 10 4.4 18 (4) 12 seconds, 482 C 100 2. 0:1 It) 4. 7 20 (5) None 50 100 2. 0:1 Porous and weak (learning occurred) R1 is the rate of the feed or supply roll while It: is the rate of the take-up 0r starch roll. b TE %-Index of fiber organization wherein T is tenacity at break in grams per denier and 1s elongation in percent extension from original length at break in tensile test. An explanation of this test and its significance is given in the Textile Research Journal 36, No. 7, pp. 5934302, July 1966.
The Table I indicates the effect that thermal pretreatment followed by hot drawing have on improving the tensile strength of foamable polybenzimidazole strand material and further indicate that such properties can be achieved while working at fiber or yarn supply speeds in the order of 50 meters per minute.
It is also interesting to note that the yarn of run (5) had been previously oven dried but still foamed.
Similar results are obtained when a mono-filament polybenzimidazole is used in place of the yarn as well as when other polybenzimidazole fibers or yarns such as those specifically mentioned heretofore are used.
The principle, preferred embodiment, and mode of operation of the present invention have been described in the foregoing specification. However, it should be understood that the invention which is intended to be protected herein, may be practiced otherwise than as described without departing from the scope of the appended claims.
We claim:
1. A process for improving the tensile properties of foamable polybenzimidazole strand material, which process comprises subjecting the strand material, prior to hot drawing, to a non-degrading heat treatment at a temperature in the range of from about 50 C. below the hot drawing temperature up to the degradation temperature of the strand material for a period of time of at least about one second up to about 50 seconds, and thereafter subjecting the resulting heat treated material to a hot drawing operation conducted at a temperature in the range above about 400 C. up to the degradation temperature of the strand material for a period of time in the range of about 1 to 50 seconds and at a draw ratio above about 1.5 :1 up to that at which the strand material breaks to yield a polybenzimidazole strand material having improved tensile properties.
2. The process of claim 1 wherein the heat treatment and hot draw operations are conducted at essentially the same temperature.
3. The process of claim 1 wherein the heat treatment is conducted on a continuous basis for a period of time in the range of from about 5 to 50 seconds and the hot draw operation is conducted at a temperature in the range of from about 450 to 650 C. at a draw ratio of from above about 1.521 to 3.5:1.
4. A process for improving the tensile properties of ioamable aromatic polybenzimidazole strand material, which process comprises subjecting the strand material, prior to hot drawing, to a non-degrading heat treatment range of from about 2.0:1 to 32:1 is used in the hot drawing operation and the hot drawing residence time is from about 10 to 20 seconds.
6. The process of claim 1 wherein the strand material is a fiber.
7. The process of claim 1,wherein the strand material is a yarn having from 10 to 1000 filaments each filament having a denier of from about 1 to 20.
8. The process of claim 1 wherein the polybenzimidazole is poly-2,2'-(m-phenylene)-5,5'-bibenzimidazole,
9. A process for improving the tensile properties of foamable poly-2,2-(m phenylene)-5,5-bibenzimidazole yarn, which comprises subjecting the yarn, prior to hot drawing, to a non-degrading heat treatment on a continuous basis at essentially the same temperature as the hot draw temperature for a period of time of from about 5 to 30 seconds, and thereafter subjecting the resulting heat treated material to a hot drawing operation at a temperature in the range of from about 500 to 550 C., at a draw ratio of from about 2.011 to 3.2:1 for a period of time of from about 10 to 20 seconds to yield a yarn having improved tensile properties.
10. The process of claim 2 wherein the heat treatment is conducted on a continuous basis for a period of time in the range of from about 5 to 50 seconds and the hot draw operation is conducted at a temperature in the range of from about 450 to 650 C. at a draw ratio of from above about 1.5:1 to 3.521.
11. The process of claim 5 wherein the strand material is a fiber.
12. The process of claim 5 wherein the strand material is a yarn having 10 to 1000 filaments each filament having a denier of from about 1 to 20.
13. The process of claim 5 wherein the polybenzimidazole is poly-2,2-(m-phenylene) -5,5'-bibenzimidazole.
References Cited UNITED STATES PATENTS 3,414,645 12/1968 Morgan 264-]84 3,415,782 12/1968 Irwin et al 264205 3,441,640 4/1969 Santangelo 264203 JULIUS FROME, Primary Examiner H. MINTZ, Assistant Examiner US. Cl. X.R. 260-47; 264-345