Highly oriented polymer fiber and method for making the same

Abstract: The present invention provides a highly oriented aramid fiber with high tensile strength, high tensile modulus and low breakage elongation by spinning liquid crystal polymers such as aramids and drafting the polymer in or adjacent to a first coagulating bath. A polymer solution with concentration of 4-24 weight % is extruded as a stream (9) into a non-coagulating fluid (10), and the stream (9) is stretched in the non-coagulating fluid (10) and passed through a first coagulating bath (4) where it is drafted by using a drafting roller (3), so that the polymer concentration of the flue is increased sufficient to form fibers. As a result, a highly oriented aramid fiber with tensile strength of 1500-5000 MPa, tensile modulus of 200-500 GPa, and the % elongation of 0.8-1.4 is provided.

Description: The present invention relates to polymer fibers and a method for making the fibers from a polymer solution. More particularly, the present invention relates to improvements in the method for making polymer fibers known as "dry-jet wet spinning." Polymers can be spun into fibers having a variety of uses. In particular, liquid crystalline main chain polymers, such as poly(p-phenyleneterephthalamide), have unique physical characteristics making them useful in the production of high-strength fibers. For example, aramid fibers (fibers made from aromatic polyamides) are well known for their strength. One method of processing linear polymers into fibers is known as dry-jet wet spinning. In this procedure a solution of the polymer, commonly referred to as a "spinning dope," is extruded from a die first through a layer of non-coagulating fluid and then into a coagulating bath. While in the coagulating bath, the solvent is removed from the dope so as to form the fiber. Tension is applied to the fiber as it leaves the coagulating bath. This stretches the fiber, which improves the degree of orientation of polymer molecules in the lengthwise direction of the fiber. While processes such as dry-jet wet spinning produce fibers having good tensile strength, they result in fiber having less than optimum tensile modulus. In accordance with the present invention there is provided a process for making polymer fibers so as to improve their properties, that is, high tensile strength, high tensile modulus, and low breakage elongation. In order to achieve the above-mentioned aims, the highly oriented polymer fiber of the present invention is characterized in that the tensile strength ranges from 1500 to 5000 MPa, the tensile modulus ranges from 200 to 500 GPa, and the breakage elongation ranges from 0.8 to 1.4%. In the fibers, the polymer is preferably aromatic polyamide. The tensile strength is preferably 2500-4500 MPa, and the tensile modulus ranges 200-300 GPa. It is also preferable in the fibers that stripe patterns are not seen in the direction perpendicular to the fiber axis when the orientation of the passing axes of the polarizer and the analizer of the highly oriented polyamide fibers is observed by using a polarizing microscope. It is preferable in the fibers that the fibril will not be generated substantially when the highly oriented polyamide fiber is stretched until it is broken. The method for producing the highly oriented polymer fibers of the present invention comprises the sequential steps of: a) extruding as a stream a solution of a polymer having a polymer concentration of 4-24 weight % into a first non-coagulating fluid; b) stretching the stream while in the non-coagulating flue at a spinning draft of 25-2000; c) passing the stream through a first coagulating bath in order to increase the polymer concentration by at least 2 weight % to a concentration of 20-65 weight %; d) stretching the stream at a stretch ratio 1.3-8 in a second non-coagulating fluid; and e) passing the stream through a second coagulating bath and increasing the polymer concentration in the stream sufficiently to form a fiber. In these steps, "stream" means the intermediate of the highly oriented fibers. In the method, the polymers are preferably aromatic polyamides. In the b) step, the "non-coagulating flue" indicates, for example, the area between the spinning cap and an optional point in the first coagulating bath. In the d) step, drafting is carried out inside or adjacent to the first coagulating bath. In the e) step, the polymer can be maintained to be a predetermined length, otherwise it is strained, or stretched. The highly oriented polymer fibers of the present invention are made in the above-mentioned method, and have tensile strength of 1500-5000 MPa, tensile modulus of 200-500 GPa, and breakage elongation of 0.8-1.4%. It is preferable in the process that the stream passes between a first driven roller and a cooperating press roller before stretching, and then passing over a second driven roller followed by a third driven roller. It is preferable in the process that the process of the invention further comprises a fourth driven roller that contacts the fiber in a manner to effect a tension ratio of 0.8-1.2 in the filament passing through the second coagulating bath. It is preferable that the stream is passed through the second coagulating bath at a tension ratio of 0.8-1.2. It is also preferable that the stream is passed through the second coagulating bath at a tension ratio of 0.9-1.2. The present invention includes a method for making a fiber comprising the sequential steps of: a) extruding as a stream a solution of a polymer having a polymer concentration of 4-24 weight % into a non-coagulating fluid; b) stretching the stream while in the non-coagulating fluid at a spinning draft of 25-2000; c) increasing the polymer concentration by at least 2 weight % to a concentration of 20-65 weight %; d) stretching the stream at a stretch ratio of 1.3-10, preferably 1.5-8; and e) increasing the polymer concentration in the stream sufficiently to form a fiber. The present invention also includes a polymer made by a claimed process as well as a polyamide fiber having a tensile strength of at least 1210 MPa and a tensile modulus of at least 145 GPa. FIG. 1 is a schematic representation of an apparatus for performing a first preferred embodiment of the process for the present invention. FIG. 2 is a schematic representation of an apparatus for performing a second preferred embodiment of the process for the present invention. FIG. 3 is a graph plotting the tensile strength as a function of spinning draft and stretch ratio for various fiber samples. FIG. 4 is a graph plotting the tensile modulus as a function of spinning draft and stretch ratio for various fiber samples. FIG. 5 is a graph plotting the tensile strength as a function of the temperature of the first coagulating bath for various fibers. FIG. 6 is a graph plotting the tensile modulus as a function of the temperature of the first coagulating bath for various fibers. FIG. 7 is a graph plotting the tensile strength as a function of the temperature of a drafting roller for various fibers. FIG. 8 is a graph plotting the tensile modulus as a function of the temperature of a drafting roller for various fibers. FIG. 9(A) is a traced scanning electron micrograph of the broken end of a fiber made according to the present invention (100×) and FIG. 9(B) is a traced scanning electron micrograph that is further magnified (700×). FIG. 10(A) is a traced scanning electron micrograph of the broken end of a commercial fiber (100×) and FIG. 10(B) is a traced scanning electron micrograph that is further magnified (700×). FIG. 11 is a schematic representation of an apparatus for performing a third preferred embodiment of the process according to the present invention. FIG. 12 is a schematic representation of an apparatus for performing a fourth preferred embodiment of the process for the present invention. FIG. 13 is a schematic representation of an apparatus for performing a fifth preferred embodiment of the process according to the present invention. FIG. 14 is a schematic representation of an apparatus for performing a sixth preferred embodiment of the process for the present invention. FIG. 15 is a schematic representation of an apparatus for performing a seventh preferred embodiment of the process for the present invention. FIG. 16 is a graph plotting the relationship between the tensile strength and tensile modulus of samples of the present invention and those of commercial fibers. FIGs. 17(A)-(C) are representations of position of a passing axis of polarizer (P) and analizer (A) observed by using a polarizing microscope. FIGs. 18(A)-(C) are traced photographs of samples of the present invention, which are observed by using a polarizing microscope (1000×). FIGs. 19(A)-(C) are traced photographs of commercial aramid fibers, which are observed by using a polarizing microscope (1000×). The method for the present invention is useful in making fibers from any polymer that is capable of forming a spinnable solution. Preferably, the polymers useful in accordance with the present invention are linear polymers, especially liquid crystalline main chain polymers. Liquid crystalline main chain polymers are well known liquid crystalline polymers (see Alger, Polymer Science Dictionary, Elsevier Applied Science (1989)) in which the crystalline units are part of the main polymer chain and are linked together by either rigid links, giving a rigid polymer backbone as in aromatic polyesters, aromatic polyamides, and poly-(p-phenylenebenzo-thiazole), which are known as rigid rod polymers, or flexible links, as in copolymers of ethylene-terephthalate and p-oxybenzoate. Most preferred are the rigid rod polymers. Exemplary polymers include, for example, polyolefins such as polyethylene, polypropylene, ethylene-propylene copolymers, polyoxymethylene, and polyethylene oxide, and aromatic polyamides such as disclosed in U.S. Pat. Nos. 3,414,645, 3,767,756, 4,466,935, and 4,344,908. Preferred polymers include the aromatic polyamides (which make fibers known as aramids), which have repeating units of the formulas -NH-R-NH-, -CO-R'-NH-, or -CO-R"-CO-, wherein R, R', and R" are optionally substituted m- or p-phenylene. Examples of useful polyamides include poly(m-phenyleneisophthalamide) (also known as MPD-I), poly(p-benzamide) (also known as PBA), poly(p-phenyleneterephthalamide), poly(p-phenylene p,p'-biphenylcarboxamide), poly(p-phenylene 1,5-naphthalenedicarboxamide), poly(trans-1,4-cinnamamide), poly(-phenylene 4,8-quinolinedicarboxamide), poly(1,4-[2,2,2]-bicyclo-octylene terephthalamide), copoly(p-phenylene 4,4'-azoxybenzene-dicarboxamide/terephthalamide), poly(p-phenylene 4,4'-trans-stilbenedicarboxamide), and poly(p-phenylene acetylenedicarboxamide). Other useful polymers include polybenzazoles, which have repeating units of the Formula 1: wherein Z is a sulfur atom (known as polybenzothiazoles) or an oxygen atom (known as polybenzoxazoles). Polymers comprising isomers of the repeating units of the foregoing formulas are also useful, such as poly(benzo[1,2-d:4,5-d']bisthiazole-2,6-diyl-1,4-phenylene) (known as trans-PBT) and poly(2,5-benzoxazole) (known as 2,5-PBO). Exemplary polybenzazoles include homopolymers such as poly(benzo[1,2-d:5,4-d']bisazole-2,6-diyl) (known as cis-PBZ), poly(2,6-benzazole) (known as 2,6-PBZ), and poly(6,6'-bibenzazole-2,2'-diyl (known as 2,2'-PBZ). Useful polymers also include copolymers of aromatic polyamides and polybenzazoles, such as poly(p-phenylene benzobisthiazole). Solvents in which to dissolve the polymers so as to form spinnable solution as well as methods of forming the spinning dope are well known, such as disclosed in the aforesaid U.S. Pat. No. 3,767,756. Examples include sulfuric acid, chlorosulfuric acid, fluorosulfuric acid, poly(phosphoric acid), and mixtures thereof. In general, the concentration of the polymer in the spinning dope is 4-24 weight %, preferably 6-22 weight %. For particular polymers, polymer concentration will be governed to some extent by the viscosity requirements for forming a spinnable solution, as will be readily apparent to those of ordinary skill in the art. Techniques for preparing spinning dopes at the appropriate viscosities as well as spinning techniques useful in accordance with the present invention are also well known, such as disclosed in the aforesaid U.S. Pat. No. 3,767,756. The present invention will now be described in detail with reference to the accompanying FIG. 1, demonstrating a first preferred embodiment of the present invention. As shown in FIG. 1, the spinning apparatus includes spinning head 1, spinning tube 2, first coagulating bath 4, freely turning guide rollers 6, heated drafting roller 3, coagulating bath 5, and stretching roller 7. In operation, spinning head 1 extrudes polymer stream 9 into first non-coagulating fluid 10 contained in spinning tube 2. The stream then passes over freely turning roller 6 in first coagulating bath 4 after which the stream is taken up by drafting roller 3. The stream then passes through second coagulating bath 5 over freely turning rollers 6 and is then wound as a fiber 8 on stretching roller 7. The temperature at which the polymer solution is spun is sufficiently high to maintain the dope in a liquid state without degrading the polymer. Preferably, the polymer solution is spun at a spinning-head temperature of 70-100°C, more preferably 70-90°C. The temperature of the non-coagulating fluid 10 and the distance in the non-coagulating fluid through which the stream 9 passes are determined so that sufficient stretching of the stream in its longitudinal direction occurs while the stream is in the non-coagulating fluid. This stretching is important to ensure that the polymer molecules in the stream are properly oriented. Accordingly, the temperature must be sufficiently warm so that the polymer molecules can reorient themselves freely within the stream. Preferably this temperature is 40-110°C, more preferably, 40-100°C, most preferably, 60-95°C. The distance that the polymer stream is in the first non-coagulating fluid depends on the initial diameter of the stream, that is, the diameter of the spinning orifice. The larger the spinning orifice the longer the distance needed to stretch the stream to a sufficient extent to reorient the polymer molecules. Preferably, the distance is 5-50 cm, more preferably 10-35 cm, most preferably 15-35 cm. Useful non-coagulating fluids are air, toluene, or heptane. Other useful non-coagulating fluids will be readily apparent to the skilled artisan. Preferably, the non-coagulating fluid is introduced at the bottom of spinning tube 2 so as to flow upward in the tube and then exit the spinning tube through appropriate apertures in the top of the spinning tube. This type of circulation is preferred in order to help prevent saturation of the non-coagulating fluid with solvent vapors. In the first coagulating bath 4, solvent is removed from the stream 9 to increase the concentration of the polymer in the stream by at least 2 weight %, preferably by at least 10 weight %, to obtain a concentration of 15-70 weight %, preferably 30-40 weight %, in the polymer stream. Accordingly, the material in the first coagulating bath is any material known for use in coagulating baths, such as disclosed in the aforesaid U.S. Pat No. 3,767,756. The temperature of the first coagulating bath and the distance through which the stream travels therein must be sufficient to effect the necessary polymer concentration. The preferred temperature of the first coagulating bath is 5-50°C. The distance through which the stream passes in the first coagulating bath must also be sufficient to effect the needed polymer concentration at the temperature of the bath, in general, streams of larger diameter needing longer distances. Variables such as the size of the extrusion orifice in the spinning head 1, the extrusion rate of the spinning head, and the speed of the drafting wheel 3 are determined to ensure a sufficient stretch in the stream 9, particularly while the stream is in the non-coagulating fluid 10. Accordingly, these variables are adjusted so that a spinning draft of 25-2000, preferably 100-2000, most preferably 150-250 is achieved. The "spinning draft" is the ratio of the velocity of the polymer stream after it leaves the first coagulating bath, which in the preferred embodiment is the same as the speed of the roller 3, to the extrusion velocity V 0 of the stream at the spinning head. Extrusion velocity V 0 is calculated according to the equation V 0 =4Q/π R 2 , where Q is the amount of the stream passing through the spinning head per unit time (extrusion rate) and R is the diameter of the spinning orifice. Preferably, the size of the spinning head orifice through which the dope is extruded is 0.3-4 mm, more preferably 0.5-1 mm. Extrusion rates preferably vary from 0.1-3 g/min, more preferably 0.25-1.5 g/min. In accordance with these parameters, the speed of the heated roller 3 is adjusted to effect the desired spinning draft. Other means for effecting the spinning draft will be readily apparent to the skilled artisan. After the concentration in the stream 9 is increased in accordance with the present invention in the first coagulating bath 4, it is necessary to additionally stretch the stream in order to more fully orient the polymer molecules in the stream in the desired direction before the stream is formed into the fiber 8 in the second coagulating bath 5. Since the stream 9 was cooled considerably in the first coagulating bath 4 in accordance with the preferred embodiment, it is preferable to raise the temperature of the stream sufficiently to enable the polymer molecules to be oriented within the stream during this additional stretching. In the presently disclosed embodiment, this is accomplished by heated drafting roller 3, which preferably raises the temperature of the stream to 15-80°C, more preferably 20-60°C and, in some instances, 20-40°C. However, other means for raising the temperature of the stream will be readily apparent to the skilled artisan. If the temperature of the first coagulating bath is higher than 15°C, it is not absolutely necessary to raise the temperature of the stream for the second stretching, although an increase by at least 5°C might be advantageous in some instances to permit a greater stretch. In the first embodiment, after the first coagulating bath, additional stretching is applied to the stream while in a second-non-coagulating fluid (in the present embodiment air, but other non-coagulating fluids are contemplated such as disclosed above for the first non-coagulating fluid) by adjusting the stretching roller 7 to a speed faster than that of heated drafting roller 3. This makes the speed of the fiber 8 at the roller 7 greater than the speed of the stream 9 at the roller 3, which stretches the stream, particularly in the area between the roller 3 and the second coagulating bath 5, that is, while the stream is in the second non-coagulating fluid. Accordingly, the speed of the stretching roller 7 is adjusted to effect a stretch ratio of 1.3-8, more preferably 1.5-3. Other means for stretching the stream to effect the desired stretch ration will be readily apparent to the skilled artisan. After passing over heated roller, the stream 9 then passes over freely turning guide roller 6 into second coagulating bath 5. Second coagulating bath is for the purpose of removing the remaining solvent from the stream and forming the fiber 8, a fiber being formed when the concentration of the polymer in the material is at least 85 weight %, preferably 85-98 weight %. The forming of fibers using such coagulating baths is well known, such as disclosed in the aforesaid U.S. Pat No. 3,767,756. Accordingly, parameters of the second coagulating bath, such as composition and dipping length, will be readily apparent to the skilled artisan. For example, both organic and aqueous fluids, such as methanol, methylene chloride, as well as aqueous solutions of sulfuric acid or ammonium hydroxide, are useful. The dipping time in the second coagulating bath must be sufficient to effect the desired polymer concentration. Preferably, the dipping time is 1-10 seconds, more preferably 1-5 seconds. The temperature of the second coagulating bath is preferably 5-80°C, more preferably 20-75°C. Higher temperatures within this range will effect additional stretch of the fiber and will aid in washing out of residual solvent at the end of the bath. After coagulation, the formed fiber can be washed, for example, in a 75°C hot water bath or spray, to remove additional solvent or otherwise treated, for example by further stretching, in accordance with known procedures. Also, it is preferred that the fiber is heat treated in accordance with known procedures, such as disclosed in the aforesaid U.S. Pat. No. 3,767,756. Preferably, heat treatment is performed by heating the fiber as it passes between a feed roll and take-up roll. Preferably, heat treatment is carried out at between a 15% stretch (speed of take-up roll 15% faster than speed of feed roll) and 10% shrinkage (speed of take-up roll 10% slower than speed of feed roll), more preferably between 3% stretch and 5% shrinkage, most preferably 0% shrinkage (speed of take-up roll and feed roll the same). In accordance with the present invention, stretching the fiber 3% or 15% during heat treatment results in a fiber having higher tensile modulus than if heat treated at 9%, but tensile strength at 3% or 15% stretch is lower than at 0%. For fiber heat-treated and shrunk at 5%, the tensile modulus is lower than 0%, but tensile strength is greater. Accordingly, heat treatment is preferably carried out in accordance with the present invention at 450-600°C, more preferably 550-600°C, for 1-10 seconds, more preferably 2-5 seconds. Heat-treated rigid-rod polymer fibers made in accordance with the present invention exhibit improved tensile modulus when compared with the same heat-treated fibers made by prior art processes. In accordance with the first preferred embodiment of the present invention as disclosed hereinabove, there are several tension zones, T 0 , T 1 , T 2 , and T 3 , as shown in FIG. 1. In general, when the variables are adjusted in accordance with the present invention, the relative tension T effected in the stream in the zones follows the formula T 3 >T 2 >>T 1 >T 0 . T 4 is a zone where the spinning solution (stream) 9 spun from the spinning cap is drafted until the stream reaches the guide roller 6 inside the first coagulating bath 4. A second preferred embodiment is shown in accompanying FIG. 2. As shown in FIG. 2, the spinning apparatus includes spinning head 1, spinning tube 2, second coagulating bath 5, freely turning guide rollers 6 and 12, drafting roller 3, press roller 11, and stretching roller 7. In operation, spinning head 1 extrudes polymer stream 9 into non-coagulating fluid 10 contained in spinning tube 2. The stream then pass over freely turning roller 6 in coagulating bath 5 after which the stream passes between cooperating drafting roller 3 and press roller 11. The stream then passes through coagulating bath 5 over freely turning roller 12 and is then wound as a fiber 8 on stretching roller 7. Press roller 11 and drafting roller 3 cooperate in such a manner as to control the speed of the stream passing therebetween. In this way, the proper spinning draft is achieved. The distance between the roller 6 and drafting roller 3 is adjusted so that the proper concentration of the polymer in the stream is achieved in order that the stretch that is imparted to the stream after passing between rollers 3 and 11 occurs at the proper polymer concentration. That is, the concentration is increased by at least 2 weight %, preferably by at least 10 weight %, to obtain a polymer concentration of 15-70 weight %, preferably 30-40 weight %, in the polymer stream. The speed of stretching roller 7 is adjusted with relation to the speed of cooperating rollers 3 and 11 in order to effect the proper stretch ratio. That is, the speed of the stretching roller 7 is adjusted to effect a stretch ratio of 1.3-8, more preferably 1.5-3. The temperature of the coagulating bath 5 in the second preferred embodiment, as well as its composition, can be the same as that for the second coagulating bath in the first preferred embodiment. Other variables and conditions can be the same as disclosed for the first preferred embodiment. FIGs. 9(A) and (B) are electron micrographs of a broken end of a poly(p-phenyleneterephthalamide) fiber made in accordance with the present invention. As demonstrated in FIGs. 9 (A) and (B), the individual fibrils that make up the fiber are all arranged in the longitudinal direction of the fiber. Also, as observed in FIGs. 9(A) and (B), when the fiber is broken the individual fibrils do not split longitudinally. This is believed due to the existence of an increased number of tying molecules between the individual fibril strands, which is believed to improve the modulus of the fiber. FIG. 9(A) is a traced scanning electron micrograph of the broken end of a fiber made according to the present invention (100×) and FIG. 9(B) is a traced scanning electron micrograph that is further magnified (700×). For comparison, FIGs. 10(A)-10(C) show the broken end of a poly(p-phenyleneterephthalamide) fiber provided by Du Pont with a trademark of "KEVLAR 49". FIG. 10(A) is a traced scanning electron micrograph of the broken end of this commercial fiber (100×) and FIG. 10(B) is a traced scanning electron micrograph that is further magnified (700×). FIGs. 11, 12, and 13 represent various embodiments, which improve the uniformity of the fibers thus produced by incorporating a press roller 11 in cooperation with driven roller 13. By separating filament stretching from the final filament coagulation, as exemplified in FIGs. 11, 12, and 13, fiber strength and modulus are improved. In the embodiment of FIG. 11, the polymer filament proceeds through the quasi-coagulating bath guided by free rollers 6 where the filament shrinks giving additional stretch to the filament between the nozzle 1 and the first free roller 6. As shown in FIG. 11, the first free roller 6 in the quasi-coagulating bath 4 is replaced by a combination of driven roller 13 and pressing roller 11, which avoids friction fluctuations that may be caused by the first free roller 6 seen in FIGs. 11 and 12. Avoiding friction fluctuations avoids draft (stretch) fluctuations caused thereby, effecting a more uniform fiber. Also, whereas the embodiment in FIG. 1 involves stretching in the coagulating bath 5, the embodiment in FIGs. 11 and 12 separates the stretch process from the coagulating bath 5 by completing the stretch at the quasi-coagulation stage. Allowing very little, or no, stretch in the final coagulating bath (by previously completing stretching) reduces shrinkage of the fiber in the bath. Shrinkage in the bath-decreases molecular orientation in the fiber, with a resulting decrease in fiber strength and modulus. In the FIG. 12 embodiment, the quasi-coagulated filament 14 is stretched at a preferred stretch ratio of 1.3-8 between driven rollers 23 and 33. The temperature between rollers 23 and 33 is adjusted to 0-90°C, preferably 20-60°C, by heater 15, at least 5°C greater than the temperature of the first coagulating bath, optimally varied depending on the polymer concentration in the quasi-coagulated filament. Therefore, reducing fiber stretch in the bath increases fiber strength and modulus. For example, at a polymer concentration in the quasi-coagulated filament of about 30-40 weight %, the preferred temperature range is 0-70°C, whereas, at a 50-60 weight % concentration, the preferred temperature is 0-90°C. Following stretching, the filament is coagulated between driver rollers 33 and 43, passing through coagulating bath 5 over free rollers 6. Preferably, the polymer concentration in filament 14 entering coagulating bath 5 is 25-40 weight % and the relative speeds of rollers 43/33 (ratio R 3 /R 2 ) is 0.8-1.2, more preferably 0.9-1.2, most preferably 1.0-1.1. Optimum tension in coagulating bath 5 depends on the orientation of the polymer molecules in the filament produced by the stretch between driven rollers 23 and 33. During stretch between rollers 23 and 33, polymer concentration in the filament remains constant (for example 30 weight %), the filament is still soft, deformation is easy, and stretch is effective to obtain a high molecular orientation. During stretch between roller 33 and 43, polymer concentration is increasing all the way. As the polymer concentration increases to 50-60 weight %, molecular orientation may still increase. But, at 70 weight % or more, molecular orientation is almost frozen. Even though molecular orientation no longer increases, fiber properties improve under high-tension coagulation. During final coagulation, the filament shrinks if the filament tension is too low. That is, orientation of molecules is decreasing all the way through the final coagulating bath until the filament reaches complete coagulation. When filament tension in the final coagulating bath is sufficiently high, even without stretch in the bath (R 3 /R 2 =1), molecular orientation is increased due to volume decrease of the filament during coagulation. FIGs. 13 and 14 represent variations on the concept shown in FIG. 12. In FIG. 13, rather than include a press roller in opposition to the driven roller in the quasi-coagulation bath, the spun fiber is wound at least one turn around roller 23 in quasi-coagulation bath 4 followed by passing through air heated by heater 15 after it leaves the quasi-coagulation bath 4, followed by passing over driven roller 33 and-then into coagulation bath 5 and then over driven roller 43. Preferred roller speeds, heater temperature and polymer concentration in filament entering coagulating bath are the same as in FIG. 12. The polymer concentration in the quasi-coagulated filament 14 depends on dipping time, filament diameter, and the temperature in the quasi-coagulating bath. The following Table 1 illustrates the variation in polymer concentration as a function of dipping, where the starting filament diameter is 62 µ m, and the quasi-coagulating-bath temperature 5°C. Dipping time (sec.) 1 2 3 4 5 Polymer concentration after quasi-coagulation by weight % 12 18 26 34 46 FIG. 14 is a variation of FIG. 13 wherein the coagulation bath 4 is replaced by a water shower effected by shower nozzle 16, by which water is sprayed on the roller 23. In FIGs. 13 and 14, polymer concentration in the filament is controlled by either dipping time in the quasi-coagulation bath 4 or resident time on the roller 23. In situations where the speed of driven roller 23 is high, that is, greater than 20 m/min., use of sulfuric acid in either the shower or quasi-coagulation bath is preferred, for example, a 60 weight % sulfuric acid solution effects a 40 weight % polymer concentration in the filament. Preferably, the concentration of sulfuric acid (in the water bath) is 50-85 weight %, more preferably 60-70 weight %. The foregoing descriptions of preferred embodiments are provided by way of illustration. The practice of the present invention is not limited thereto and variations therefrom will be readily apparent to the skilled artisan without deviating from the spirit of the present invention. The following, non-limiting examples are provided to further illustrate the present invention. All parts and percentages in the examples are by weight unless indicated otherwise. EXAMPLES 1-6 Fibers are prepared from a high molecular weight rigid-rod polymer in accordance with the present invention at various stretching concentrations (that is, concentration of polymer after leaving the first coagulating bath). Using the apparatus described in FIG. 1, the following conditions are employed; spinning solution is poly(p-phenyleneterephthalamide) having an inherent viscosity of 5.2 (obtained from AKZO, Netherlands) dissolved in 99.5 % H 2 SO 4 at a concentration of 7 weight %, extrusion rate is 0.1 g/min.; spinning-nozzle orifice diameter is 1 mm; spinning tube-length is 7 cm; temperature of spinning solution, nozzle temperature, and spinning tube temperature are 80°C; first coagulating bath is water at a temperature of 5°C; temperature of heated drafting roller is 20°C; the second coagulating bath is water at a temperature of 20°C; spinning draft is 150. The dipping length in the first coagulating bath is adjusted between 17-70 cm in order to achieve the desired stretching concentration. The dipping length in the second coagulating bath is accordingly adjusted between 30-90 cm to obtain a fiber from the polymer stream. The stretch ratio is adjusted to 0.5 × maximum stretch ratio. The "maximum stretch ratio" is determined by passing the stream immediately from the drafting roller to the stretching roller and increasing the speed of the stretching roller until the fiber breaks. The resulting polymer concentrations are reported in the following Table 2. Example Polymer Concentration (%) Dipping Length (cm) Stretch Ratio Dipping Length (cm) 1 12 17 1.65 55 2 24 26 1.8 50 3 33 36 1.8 50 4 41 44 1.7 45 5 47 53 1.5 45 6 56 70 1.3 40 Following the second coagulating bath the fiber is washed with a hot-water spray (75°C) to remove residual acid. The final fiber is then heat treated at 0% shrinkage and 550°C for 5 seconds. The fibers are tested for tensile modulus and tensile strength in accordance with ASTM Method D 3379-75 (1975), and % elongation is the difference between the fiber length at breakage and the original length divided by the original length. The results are recorded in the following Table 3. Example Denier Tensile Strength MPa Tensile Modulus GPa % Elongation 1 3.7 2850 240 2.0 2 3.3 3270 340 1.6 3 3.3 3420 390 1.4 4 3.5 3320 420 1.4 5 3.6 3180 416 1.4 6 4.4 2840 383 1.5 Examples 1-6 are repeated with results as reported in the following Table 4. Example Denier Tensile Strength MPa Tensile Modulus GPa % Elongation 1A 3.7 2850 167 1.7 2A 3.3 3270 251 1.3 3A 3.3 3420 310 1.1 4A 3.5 3320 301 1.1 5A 3.6 3180 289 1.1 6A 4.4 2840 236 1.2 As demonstrated in Tables 3 and 4, the fibers made according to the present invention have an excellent balance of tensile strength and tensile modulus. EXAMPLES 7-12 A series of fibers are made by redissolving an aramid fiber (KEVLAR 29 available from E.I. du Pont de Nemours Co.) in 95% H 2 SO 4 and spinning the resulting solution to make fibers in accordance with the present invention at various stretching concentrations. Conditions followed are as in Examples 1-6 except as modified below. As in Examples 1-6, stretching concentration is varied by varying the dipping length of the stream in the first coagulating bath between 5-40 cm, and the dipping length in the second coagulating bath is accordingly adjusted such that the final fibers have the same polymer concentration and roughly similar denier. Conditions are recorded in the following Table 5. Example Polymer Concentration (%) Dipping Length (cm) Stretch Ratio Dipping Length (cm) 7 11 5 1.8 60 8 17 12 2.1 55 9 27 23 2.4 50 10 36 35 2.2 50 11 44 45 1.9 45 12 52 60 1.6 45 The fibers are tested for tensile modulus and tensile strength in accordance with ASTM Method D 3379-75 (1975). The results are recorded in the following Table 6. Example Denier Tensile Strength MPa Tensile Modulus GPa % Elongation 7 3.3 2780 115 2.4 8 2.8 2940 140 2.1 9 2.5 2980 186 1.6 10 2.7 2810 234 1.2 11 3.1 2520 210 1.2 12 3.7 2140 194 1.1 As in Examples 1-6, the fibers made in accordance with the present invention have an excellent balance of tensile strength and tensile modulus. EXAMPLE 13 Samples of fiber are made in accordance with the present invention. at different spinning drafts. Spinning conditions, using the apparatus described hereinabove, are follows; spinning solution is poly(p-phenyleneterephthalamide) (as in Examples 1-6) dissolved in 99.5% H 2 SO 4 ; concentration of polymer in spinning solution is 20 %; temperature of spinning solution, spinning nozzle, and spinning tube are 85°C; nozzle-orifice diameter is 1 mm; extrusion rate is 0.15 g/min; spinning tube length is 7 cm; first coagulating bath is water at a temperature of 5°C; dipping length in first coagulating bath is 5 cm; polymer concentration at drafting roller is 40 %; temperature of drafting roller is 20°C; second coagulating bath is water at a temperature of 20°C; dipping length in the second coagulating bath is 60 cm. For each of three spinning drafts (50, 100, 150), samples are made using various stretch ratios. For each sample, tensile strength and tensile modulus are determined as in Examples 1-6. The various stretch ratios and results are recorded in FIGs. 3 and 4. As seen in FIG. 3, the fiber of spinning draft 150 and stretch ratio 1.8 (maximum stretch ratio × 0.6 for spinning draft 150) has a higher tensile strength than the fiber of spinning draft 50 and stretch ratio 2.7 (maximum stretch ratio for spinning draft 50). As seen in FIG. 4, the tensile modulus of low draft/high stretched fiber is higher than for high draft/low stretched fiber; but, the difference is not as marked as in tensile strength. EXAMPLE 14 Samples of fiber are made in accordance with the present invention using different first-coagulating-bath temperatures. Using the apparatus described hereinabove, spinning conditions are as follows: spinning solution is poly(p-phenyleneterephthalamide) (as in Examples 1-6) dissolved in 99.5% H 2 SO 4 ; concentration of polymer in spinning solution is 20%; temperature of spinning solution, spinning nozzle, and spinning tube are 85°C; nozzle orifice diameter is 1 mm; extrusion rate is 0.15 g/min; spinning tube length is 7 cm; first coagulating bath is water at a temperature of 5°C; polymer concentration at drafting roller is 40 %; spinning draft is 100; temperature of drafting roller is 20°C; second coagulating bath is water at a temperature of 20°C; dipping length in the second coagulating bath is 60 cm; stretch ratio is 2.2. The temperature of the first coagulating bath is varied and the tensile strength and tensile modulus determined as in Examples 1-6. The various temperatures and results are recorded in FIGs. 5 and 6. As seen in these figures, fiber properties are better at lower temperatures. EXAMPLE 15 Samples of fiber are made in accordance with the present invention using different drafting roller temperatures. Using the apparatus described hereinabove, spinning conditions are the same as in Example 12, with the temperature of the first coagulating bath at 5°C, except that the stretch ratio is 0.5 × the maximum stretch ratio, which is determined as in Examples 1-6. The temperature of the drafting roller is varied and the tensile strength and tensile modulus determined as in Examples 1-6. The various temperatures and results are recorded in FIGs. 7 and 8. As seen in the figures, tensile modulus increases markedly as the temperature increases, then decreases sharply as the temperature is increased further. The temperature of the drafting roller has a similar effect on tensile strength; but, both the increases and decrease are not as marked. EXAMPLE 16 Using the apparatus as shown in FIG. 12, tests are conducted to determine the effects on fiber properties of the relative speeds of driven roller 43/33 (the ratio R 3 /R 2 ) as opposed to the relative speeds of driven rollers 33/23 (the ratio R 2 /R 1 ). Other conditions are as described in the preceding Examples 1-6. Test results are reported in the following Table 7. Fiber Properties Based on Variations of R 2 /R 1 and R 3 /R 2 R 2 /R 1 R 3 /R 2 0.8 0.9 1.0 1.1 1.2 1.3 Tensile Strength (MPa) 1.8 1620 2720 3210 3580 ** ** 1.4 2060 2620 3010 3220 ** Tensile Modulus (GPa) 1.8 180 226 237 270 ** ** 1.4 * 187 208 231 248 ** % Elongation to Rupture 1.8 0.9 1.20 1.35 1.33 ** ** 1.4 * 1.1 1.25 1.30 1.29 ** Filament could not be passed through coagulating bath, since maximum filament shrinkage in the bath without tension is less than R 3 /R 2 . Filament is broken. Under the same ratio R 2 /R 1 at the same polymer concentration in the quasi-coagulated filament, R 2 /R 3 has a large influence on fiber properties. When R 3 /R 2 = 1.2 and R 2 /R 1 =1.8, filament tension in coagulating bath 5 increases, and the filament cannot withstand the tension and breaks in the coagulating bath. At R 2 /R 1 = 1.4, when R 3 /R 2 is 0.8 (or less), filament tension in the coagulating bath becomes 0, and the filament cannot be passed through the coagulating bath. When R 3 /R 2 passes a level sufficient to pass the filament through the coagulating bath, filament tension is produced by the resultant shrinking of the filament in the bath. Limitations on R 3 /R 2 apparently depend on the orientation of the polymer molecules in the filament (as it leaves the driven roller 33), which is produced by the stretch between driven rollers 23 and 33. When R 2 /R 1 =1.8 and the stretch temperature is 20°C, R 3 /R 2 = 0.8 is the maximum shrinkage ratio; that is, maximum filament shrinkage occurs in the final coagulation bath, and the polymer in the filament is less oriented compared with the orientation in the filament just after leaving driven roller 33. In the case of R 2 /R 1 = 1.4 (medium stretch), molecular orientation in the filament at roller 33 is lower than in the case of R 2 /R 1 =1.8 (maximum stretch). At R 2 /R 1 = 1.4, the maximum stretch of R 3 /R 2 (1.2) is higher than in the case of R 2 /R 1 = 1.8 ; however, tensile strength and modulus in the final fiber are lower in the case of R 2 /R 1 =1.4 compared with the case of R 2 /R 1 =1.8. Thus, higher molecular orientation at roller 33 effects a correspondingly higher tensile strength and modulus in the final fiber. EXAMPLE 17 Using the apparatus as shown and described in FIG. 12, the effects on fiber properties are determined as a function of stretch temperature (as regulated by heater 15) and polymer concentration (in filament at roller 23). Using the procedures in the previous example, the effects of heater 15 temperature are determined (polymer concentration = 30%; R 2 /R 1 =1.1). Effects of the heater temperature on fiber properties are shown in the following Table 8. Stretch-Temperature Effect Heater temperature °C 20 40 60 70 Maximum R 2 /R 1 1.8 2.4 2.6 2.2 Tensile strength MPa 3580 3690 3710 2560 Tensile modulus GPa 270 278 285 203 Elongation (%) 1.33 1.31 1.30 1.26 Heater temperature of 60°C effects maximum tensile strength and modulus. At 70°C, maximum stretch (R 2 /R 1 ) obtainable decreases to 2.2, because visco-elastic value of the filament becomes too low; molecular entanglements (proportional to visco-elastic value) at stretch being too low to obtain a higher stretch. Since less stretch (less molecular orientation) is obtained, the tensile strength and modulus at 70°C are lower than in the case of 60°C. When the procedure is repeated at a polymer concentration of 50%, optimum heater temperature is 80°C; but, because the visco-elastic value is high, the maximum stretch (R 2 /R 1 ) obtainable is 1.8, which is less than obtainable at a polymer concentration of 30% (2.6). Accordingly, tensile strength and modulus at a polymer concentration of 50% (80°C stretch) are lower than in the case of a polymer concentration of 30% (60°C stretch). The results above indicate that, at a given polymer concentration, increasing stretch temperature up to a point has a positive effect on fiber properties. On the other hand, increasing the pre-stretch polymer concentration above a certain level has negative effect on fiber properties. EXAMPLE 18 Using the procedures and optimum parameters determined in the previous Examples, fiber properties are measured as a function of coagulation tension (R 3 /R 2 ) and reported in the following Table 9. Coagulation-Tension Effect R 3 /R 2 0.7 0.8 0.9 1.0 1.025 1.05 Tensile Strength (MPa) 1210 2200 3220 3710 3035 2888 Tensile Modulus (GPa) 145 178 242 285 275 242 Elongation (%) 0.83 1.23 1.33 1.30 1.19 1.10 Filament broke at R 3 /R 2 = 1.1. The properties shown demonstrate the positive effect of providing a coagulation tension between driven rollers 33 and 43 resulting from a ratio R 3 /R 2 of about 1.0-1.1 (representing an increase in the fiber tension over that approximately corresponding to T 3 in the apparatus shown in FIG. 1). EXAMPLE 19 FIG. 15 is a schematic representation of an apparatus for performing a preferred embodiment of the process for the present invention. The device shown in Example 19 is distinguishable from those in the former Examples in that driven rollers (60, 61, 62, and 63) are used. In FIG. 15, 64 is a draft zone and 65 is a draft fiber. The spinning solution is (like in Examples 1-6) poly(p-phenyleneterephthalamide) dissolved in 99.5% H 2 SO 4 . The polymer concentration in the spinning solution is 20 %. The temperature of the spinning solution, the spinning nozzle and the spinning tube is 85°C, and the diameter of the nozzle orifice is 1 mm. The extrusion rate is 0.15 g/minute, the spinning tube length is 7 cm, the H 2 SO 4 concentration in the first coagulating bath ranges 50-70 weight % (aqueous solution), the dipping time is 1.4 second, and the temperature is 5°C. The spinning draft is 100. In the draft rollers, the polymer concentration is 40 %, the temperature of the drafting roller 62 is 20°C, the temperature of the second coagulating bath is 20°C, the dipping depth in the second coagulating bath is 60 cm, the stretch ratio in the draft zone is 3.6-4.33 (R 2 /R 1 , where R 1 is the peripheral speed of the driven roller 61, R 2 is the peripheral speed of the driven roller 62). The relationship between the peripheral speed of the driven roller 63 (R 3 ) and the peripheral speed of the driven roller 62 (R 2 ) was R 3 / R 2 =1.025. The relationship between the peripheral speed of the driven roller 60 (R 0 ) and the peripheral speed of the driven roller 61 (R 1 ) was R 1 / R 0 =1.00. The test condition is shown in the following Table 10. Test No. 1 st coagulating bath H 2 SO 4 concentration (aqueous solution) Stretch ratio (R 2 /R 1 ) Drafting time (second) 2 nd coagulating bath drafting time (sec.) Tensile strength (MPa) Tensile modulus (GPa) 2043 50 % 3.60 0.45 1.23 3670 412 2042 60 % 4.00 0.40 1.03 3420 273 2049 70 % 4.33 0.37 0.95 3360 204 FIG. 16 shows the relationship between the tensile strength and the tensile modulus of the obtained aramid draft fiber. In FIG. 16, the diameter of the single fiber is indicated in the parenthesis attached to the sample's test Nos. The data of the commercial KEVLAR 29, 49 and 149 are also shown in this figure. It is clearly shown that the tensile modulus of the aramid drafting yarn of the present invention is better than that of the commercial ones. Next, the aramid draft fibers of the present invention and a commercial KEVLAR 49 were observed under a polarizing microscope. A long aramid fiber (single fiber) that spun sufficiently was positioned at the center surface of a horizontally-positioned glass slide (76 × 26 × 1 mm), so that the direction of the fiber was parallel to the long side. The fiber was fixed with adhesive tape on the glass slide at one lengthwise end of the slide. A weight of 10g was tied at the other end of the fiber, and the slide was held up vertically while keeping the fixed end above so as to provide tension of 10g to the fiber. Maintaining the state, the fiber was fixed with adhesive tape at the bottom end of the glass slide surface so that the fiber was parallel to the long side of the slide. A cover glass (24 × 24 × 0.12 mm) was put on the center of this slide glass (the fiber was positioned on the surface), and the ends of the cover glass at the opposite of the fiber (the long sides of the glass slide) were fixed with adhesive tape on the glass slide. The fiber on the cell adjusted in this way was observed by a wet method using a polarizing microscope having an eyepiece with a magnifying power of 10, an object lens with a magnifying power of 100 (the total magnifying power of 1000). The observation was carried out at three coordinations shown in FIGs. 17(A)-(C). FIG. 17(A) shows a parallel polarizing system where the fiber axis and the passing axes of the polarizer (P) and the analizer (A) agree with each other. In FIG. 17(B), the fiber axis and the passing axis of the polarizer agree. And FIG. 17(C) shows opposite rectangular polarizing system where the fiber axis and the polarizer axis make 45 ° angle. FIG. 17(A) and 17(B) compensate with each other considering the contrast of field of view. The polarizing microscopic views were photographed. FIGs. 18(A)-(C) show the traced polarization microscopic pictures of the single fiber 50 (test No. 2049) of the present invention, and FIGs. 19(A)-(C) show the single fiber 51 of a commercial KEVLAR 49. Ten divisions on a scale indicate about 1 micron. All of the single fibers obtained in the present invention are in the substantially uniform dark field in the rectangular polarizing system (B) (it means the bright field in (C)), that is, the polymer molecular axis is oriented substantially parallel to the fiber axis. In the KEVLAR 49 single fiber, an area that is a little bright exists in the fiber axis direction around the center axis of the fiber in the rectangular polarizer system (B), and many stripe patterns are clearly observed. KEVLAR 29 shows similar result. In FIG. 19(C) with a brighter field, stripe patterns are also observed though they are not so clear as (B). These observation results show that the polymer molecular axes of a commercial KEVLAR single fibers are oriented generally to the fiber axis, but they are not completely parallel to the fiber axis; the orientation of the polymer deviates partially from the fiber axis direction. The reason of this phenomenon has not found out yet. A deviation formed during spinning might be fixed, or it might appear during a process of relaxing orientation of the polymer chain in a liquid crystal state (it does not appear during spinning for the high orientation) and be fixed. In any case, the single fiber of the present invention is formed with polymer chains that are fixed in an oriented state further uniformly compared to commercial KEVLAR fibers.

Claims: A highly oriented polymer fiber having a tensile strength of 1500-5000 MPa, a tensile modulus of 200-500 GPa, and a % elongation of 0.8-1.4. The highly oriented polymer fiber of claim 1, wherein the highly oriented polymer is aromatic polyamide. The highly oriented polymer fiber of claim 1 or 2, wherein the tensile strength is 2500-4500 MPa, and the tensile modulus is 200-300 GPa. The highly oriented polymer fiber of any one of claims 1-3, wherein no stripe patterns are seen in the direction perpendicular to the fiber axis when the highly oriented polymer fiber is observed under a polarizing microscope to examine the orientation of passing axes of the polarizer and the analizer. The highly oriented polymer fiber of any one of claims 1-4, wherein substantially no fibrils are generated when the highly oriented polymer fiber is stretched to rupture. A process for making a fiber comprising the sequential steps of: a) extruding a stream of a solution of a polymer having a polymer concentration of 4-24 weight % into a first non-coagulating fluid; b) stretching the stream while in the non-coagulating flue at a spinning draft of 25-2000; c) passing the stream through a first coagulating bath to increase the polymer concentration by at least 2 weight % to a concentration of 20-65 weight %; d) stretching the stream at a stretch ratio of 1.3-8 in a second non-coagulating fluid; and e) passing the stream through a second coagulating bath to sufficiently increase the polymer concentration in the stream to form the fiber. The process of claim 6 wherein the polymer is aromatic polyamide. The fiber made according to the process of claim 6, having a tensile strength of 1500-5000 MPa, a tensile modulus of 200-500 GPa, and a % elongation of 0.8-1.4. The process of claim 6 further comprising passing the stream between a first driven roller and a cooperating press roller before stretching the stream, effected by passing the stream over a second driven roller followed by a third driven roller. The process of claim 9 further comprising a fourth driven roller that contacts the fiber in a manner to effect a tension ratio of 0.8-1.2 in the filament passing through the second coagulating bath. The process of claim 6 wherein the stream is passed through the second coagulating bath at a tension ratio of 0.8-1.2. The process of claim 6 wherein the stream is passed through the second coagulating bath at a tension ratio of 0.9-1.2. The process of claim 6 wherein the fiber formed has a tensile strength of 1500-5000 MPa, a tensile modulus of 200-500 GPa, and a % elongation of 0.8-1.4.

Inventors: KAWAI KIGEN, KAWAI, KIGEN
Applicants: KANSAI RES INST KRI, KANSAI RESEARCH INSTITUTE (KRI)
Application year: 1998
Main classification: D01F6
IPC codes: D01D5, D01F6