IMPROVED PROCESS FOR POST-SPIN FINISHING OF POLYBENZOXAZOLE FIBERS

Abstract: Improved polybenzoxazole fibers are made by a process for finishing a spun and drawn dope fiber, which contains polybenzoxazole polymer and a solvent acid, comprising the steps of: (a) coagulating the dope fiber in an aqueous coagulant; (b) washing the coagulated fiber with an aqueous washing fluid for less than 72 hours under conditions such that the fiber contains no more than 8,000 ppm residual solvent acid (1 ppm = 1 part per million, by weight); (c) drying the fiber at a temperature of 120 C to 300 C until it retains no more than 2 weight percent residual moisture; and (d) heat-treating the fiber at a temperature of at least 300 C under tension.

Description: IMPROVED PROCESS FOR POST-SPIN FINISHING OF POLYBENZOXAZOLE FIBERS • This invention relates to the art of making heat-treated polybenzoxazole fibers. It is known to spin and heat-treat fibers that contain polybenzazole polymer. See, for example, Wolfe, U.S. Patent 4,533,693 (August 5, 1985) at col. 166-174; Takeda, Japanese Kokai 2(1990)-84511 (published March 26, 1990); and Ledbetter etal., "An Integrated Laboratory Process for Preparing Rigid Rod Fibers from the Monomers," The Materials Science and Engineering of Rigid Rod Polymers at 253, 259-61 (Materials Research Society 1989). Ordinarily, a dope is formed containing the polymer and a solvent acid. The dope is forced through a spinneret and drawn across an air gap. It is contacted with a coagulating liquid, usually water, to coagulate the polymer and form a fiber. The fiber is washed to remove residual acid. The resulting fiber is heat-treated to improve its modulus. It has not previously been recognized that polybenzoxazole fibers can be damaged by the processes used to wash and heat-treat them after spinning. Surprisingly, the tensile strength of the damaged fiber can remain relatively high for several weeks after the fiber is manufactured, but the tensile strength decreases with the passage of time. More surprisingly, the same decrease is not observed in some closely related fibers, such as PBT. An object of the present invention is to choose washing, drying and heat-treating conditions that form a fiber capable of retaining significant tensile strength over time. The present invention is a process for finishing a spun and drawn dope fiber, which contains polybenzoxazole polymer and a solvent acid, having the steps of: (A) coagulating the dope fiber in an aqueous coagulant; (B) washing the coagulated fiber with an aqueous washing fluid until the fiber contains no more than 8,000 ppm residual solvent acid (1 ppm = 1 part per million, by weight); (C) drying the fiber; and (D) heat-treating the fiber at a temperature of at least 300°C under tension characterized in that: (1) the fiber is washed in Step (B) for less than 72 hours; and (2) the fiber is dried in Step (C) at a temperature of 120°C to 300°C until it retai ns no more than 2 weight percent residual moisture. Fibers made according to the process of the present invention retain their tensile properties well. The fibers can be used for ordinary purposes of high-strength fibers, such as in ropes, composites and other structural applications. 5 The present invention uses a lyotropic liquid crystalline dope solution that contains polybenzoxazole polymer and a solvent acid. Suitable polymers and processes to make suitable dopes containing those polymers are well-known in the art. See, for example, Wolfe, U.S. Patent 4,533,693 (August 5, 10 1985); Takeda, Japanese Kokai 2(1990)-8451 1 (published March 26, 1990); Wolfe, "Rigid-Rod Polymer Synthesis: Development of Mesophase Polymerization in Strong Acid Solutions," The Materials Science and Engineering of Rigid Rod Polymers at 83-93 (Materials Research Society 1989) and Ledbetter et al., "An Integrated Laboratory Process for Preparing Rigid Rod Fibers from the Monomers," The Materials Science and Engineering of Rigid Rod 15 Polymers at 253, 257-59 (Materials Research Society 1989). The polymer may contain AB-mer units, as represented in Formula 1(a), and/or AA/BB-mer units, as represented in Formula 1 (b) 5 K a) AB Kb ) AA/BB 5 wherein: Each Ar represents an aromatic group selected so that the polymer forms lyotropic liquid crystalline domains in the solvent acid when its concentration is above a critical concentration level. The aromatic group may be heterocyclic, such as a pyridinylene group, but it is preferably carbocyclic. The aromatic group may ■ be a fused or unfused polycyclic system, but is preferably a single six-membered ring. Size is not critical, but the aromatic group preferably contains no more than 18 carbon atoms, more preferably no more than 12 carbon atoms and most preferably no more than 6 carbon atoms. Examples of suitable aromatic groups include phenylene moieties, tolylene moieties and biphenylene moieties. Ar' in AA BB-mer units is preferably a 1 ,2,4,5-tetravalent benzene moiety or an analog thereof. Arin AB-mer units is preferably a 1,3,4-travalent benzene moiety or an analog thereof. Each DM is independently a bond or a divalent organic moiety selected so that the polymer form lyotropic liquid crystalline domains in the solvent acid when its concentration is above a critical concentration level. The divalent organic moiety is preferably an aromatic group (Ar) as previously described. It is most preferably a 1 ,4-phenyiene moiety or an analog thereof. The nitrogen atom and the oxygen atom in each oxazole ring are bonded to adjacent carbon atoms in the aromatic group, such that a f ive-membered azole ring fused with the aromatic group is formed. The oxazole rings in AA/BB-mer units may be in cis- or trans-position with respect to each other, as illustrated in 11 Ency. Poly. Sci. & Eng., supra, at 602. The polymer preferably consists essentially of one of the repeating units illustrated in Formulae 2(a)-(d). ( a) - f c j s O - . It more preferably consists essentially of at least one of the repeating units represented in Formulae 2(a) and (b), and most preferably consists essentially of the repeating unit - represented by Formula 2(a). The polymer is dissolved in a solvent acid, such as methanesulfonic acid or poly¬ phosphoric acid. The solvent acid preferably contains polyphosphoric acid. The concentration of polymer should be high enough that the dope solution contains liquid crystalline domains. 0 The concentration of polymer is preferably at least 7 weight percent, more preferably at least 10 weight percent and most preferably at least 14 weight percent. The maximum concentration of polymer in the dope is governed primarily by practical considerations, such as the viscosity of the dope. Dopes ordinarily contain no more than 30 weight percent polymer and typically contain no more than 20 weight percent polymer. When the solvent acid is poly- phosphoric acid, it preferably contains at least 80 weight percent P 2 0 5 and no more than 86 weight percent P 2 0 5 . The dope is spun to form a fiber according to known processes. Useful spinning processes are known and described in the references previously incorporated by reference. Q Useful spinning processes can also be adapted from the spinning of polybenzoihiazole and polybenzimidazole polymer dopes, and are described in numerous references, such as Tan, U.S. Patent 4,263,245 (April 21 , 1981); Ide, U.S. Patent 4,332,759 (June 1 , 1982); and Chenevey, U.S. Patent 4,606,875 (August 19, 1986). 5 The dope is forced through a spinneret and drawn across an air gap. The spinneret may contain a single hole or multiple holes. The holes may range in diameter from 50 μm to 1000 μm. They are preferably at least 75 μm and preferably no more than 500 μm. The temperature of the die and dope is preferably at least 100°C and more preferably at least 130°C. It is preferably no more than 200°C and more preferably no more than 180°C. The optimum force pushing the dope through the spinnerette varies depending upon the spinnerette and spinning conditions, and can be ascertained by persons of ordinary skill in the art. The air gap is preferably at least 1 mm and more preferably at least 5 mm. The air gap is preferably no more than 100 cm. The spin-draw ratio of the dope fiber as it is drawn across the air gap is preferably at least 1 and more preferably at least 5. The optimum spin draw ratio depends on the spin die and other conditions of spinning, but it is usually less than 1000. The spun and drawn fiber is coagulated by contacting it with an aqueous coagulant. Th coagulant may contain acid or base. Its pH is preferably at least 1.0 and more preferably at least 3.0. It is preferably no more than 12 and more preferably no more than 9. The coagulant may also contain organic diluents, but it preferably does not. The coagulant may be at any temperature at which it is not frozen (usually between 0°C and 100°C), but is preferably between 0°C and 20°C. After the fiber has been coagulated, it is washed to remove residual acid. The washing uses an aqueous washing fluid. The washing fluid may be acidic or basic, but is conveniently neutral. The pH of the washing fluid is preferably at least 3, and more preferably at least 5. The pH of the washing fluid is preferably no more than 10 and more preferably no rnore than 8. The washing fluid may be a liquid or it may be steam. Liquid washing fluids may be at any temperature from 0°C to 100°C. The temperature is preferably at least 5°C and more preferably at least 10°C. It is preferably no more than 50°C and more preferably no more than 30°C. Washing may be carried out in a single stage, or in different stages such as a brief on-line washing followed by longer static washing. In a convenient static washing technique, the fiber is taken up onto a perforated spool. Running water is continuously fed into the center of the spool, from which it passes out through the perforations and the fibers. Washing may be in static water, but is preferably in running water. The washing is continued for no more than 72 hours, but until the residual solvent acid content of the fiber is no more than 8000 ppm after washing and drying. High levels of residual solvent acid are undesirable in many end uses, but excessive washing leaves the fiber susceptible to loss of tensile strength over time, particularly if the fiber sustains other damage during the manufacturing process. The residual acid content in the fiber after washing and drying is preferably no more than 5000 ppm, more preferably no more than 2000 ppm and most preferably no more than 1000 ppm. Although it is desirable to minimize the residual solvent acid content, the washed and dried fiber usually contains some measurable concentration of solvent acid. The fiber frequently contains at least 10 ppm residual acid, more frequently at least 100 ppm residual acid and most often contains at least 800 ppm residual acid. The fiber is preferably washed for no more than 48 hours, more preferably no more than 24 hours, more highly preferably no more than 12 hours and most preferably no more than 3 hours. It is important to dry the fiber before the fiber is heat-treated. The coagulated and washed fiber usually contains more water than polymer. The fiber sustains significant damage if it is heat-treated before most of that water is removed. Preferably, the fiber is dried immediately or very shortly after washing is complete. Long storage in a wet condition contributes to instability of fiber tensile strength. The fiber must be dried at a temperature high enough to remove the water in a timely and cost effective manner, but low enough to prevent damage to the fiber. The temperature is at least 120°C, more preferably at least 130°C, more highly preferably at least 140°C and most preferably at least 150°C. The temperature of drying is preferably no more than 300°C more preferably no more than 250°C and most preferably no more than 200°C. The fiber is dried until it contains no more than 2 weight percent residual moisture. It preferably contains no more than 1 weight percent residual moisture and most preferably no more than 0.5 weight percent residual moisture. The times necessary to obtain the desired residual moisture vary widely depending upon the fiber and the conditions under which it is dried. The time is not critical, as long as the fiber reaches the required residual moisture content. The drying time in an "on- -line" drying apparatus is preferably no more than 1 hour, more preferably no more than 10 minutes and most preferably no more than 5 minutes. It is limited by practical considerations, but is seldom less than 1 second. Drying may be accomplished by known means, such as running the fiber through a tubular oven. Drying may be in a single step or in multiple steps, such as a static drying at a relatively lower temperature to remove most of the water, followed by on-line finish drying in a tubular oven at a relatively high temperature. Drying is preferably car: so out predominantly in the dark and predominantly under atmosphere that is inert with respect to the fiber under drying conditions, such as nitrogen or argon. The fiber may optionally be stored for a period of time after it is dried and before it is heat-treated. Storage is preferably in the dark, in a dry atmosphere and in an inert atmosphere.- The dried fiber is heat-treated in order to improve its tensile modulus. Heat- -treatment and processes to accomplish it are well-known. Heat-treatment is preferably carried out at a temperature of at least 300°C, more preferably at least 450°C and most preferably at least 500°C. The temperature of heat-treatment is preferably no more than 1000°C, more preferably no more than 800°C and most preferably no more than 600°C. The fiber is heat- -treated under tension. The optimal tension varies depending upon the fiber and the process in which it is heat-treated. The tension is usually between 0.1 g/d and 10 g/d, and preferably between 2 g/d and 6 g/d. The optimum time of heat-treating varies broadly depending upon the fiber and the process conditions used to heat-treat it. The time is usually at least 1 second and usually no more than 30 seconds. The atmosphere may be any which does not significantly damage the fiber. It is usually air or an inert atmosphere such as nitrogen, carbon dioxide or argon. The modulus of the heat-treated f i ber is preferably at least 10 percent higher than the tensile modulus of the non-heat-treated fiber, more preferably at least 50 percent higher and most preferably at least 100 percent higher. The resulting fibers are strong, have high modulus and retain their tensile properties well. The fiber preferably has initial tensile strength of at least 600 ksi (1 ksi = 1000 psi), more preferably at least 700 ksi and most preferably at least 800 ksi. Its tensile modulus is preferably at least 38 msi (1 msi = 1,000,000 psi), more preferably at least 40 msi and most preferably at least 45 msi. Property retention can be accurately estimated by irradiating a sample of the fiber in a HERAEUS SUN TEST CPS T " instrument using 765 watts per square meter of xenon irradiation with a quartz filter for a desired period of time, such as from 100 hours to 300 hours. The tensile strength of fiber samples is tested before and after irradiation by ordinary means, such as using an INSTRON™ tensile testing instrumentto measure the force required to break a yarn bundle of fiber. After 100 hours of irradiation under the previously described conditions the fiber preferably retains at least 75 percent of its initial tensile strength, more preferably at least 80 percent, more highly preferably at least 85 percent and most preferably at least 90 percent of its original tensile strength. The tensile strength of irradiated fibers is preferably at least 550 ksi, more preferably at least 600 ksi and most preferably at least 650 ksi. The fiber may be used in composites, strong ropes and numerous other applications. The invention is illustrated by the following examples: Illustrative Examples The following examples are for illustrative purposes only and should not be taken as limiting the scope of either the specification or the claims. Unless stated otherwise, all parts and percentages are by weight. Example 1 o A dope contained 14 weight percent cis-polybenzoxazole polymer having an inherent viscosity of about 30 dL/g dissolved in polyphosphoric acid. The dope was spun through a 340 filament spin die, drawn across an air gap and coagulated in water. The coagulated fibers were washed in water for the time shown in Table 1. They were dried in an oven for the time shown in Table 1. The moisture content of the fibers was measured after 5 drying by: (1) cutting and weighing a 0.3 g sample; (2) drying the sample for 2 hours at 250°C; and (3) reweighing the sample to determine moisture lost. Each of the fibers had the residual moisture shown in Table 1. The dried yarn was about 500 denier on average. It was heat-treated at 550°C for 10 seconds residence time under 3 g/denier tension. The initial tensile strength was measured, o and is shown in Table 1. (Tensile testing used an Instron testing machine, fibers with a twist factor of 3.5, a gauge length of 4.5" and a strain rate of 0.02/min.) The fibers were exposed to 765 watt/m 2 of 300 to 800 nm light for 100 hours in an ATLAS model Ci65A weatherometer with a xenon lamp and a borosilicate filter. The tensile 5 strength of the fiber was remeasured and shown in Table 1. The percent retention of tensile strength was calculated and shown in Table 1. Table 1 0 5 Comparative Example 2 A dope contains 14 weight percent cis-poiybenzoxazole polymer having an inherent viscosity of about 30 dL/g dissolved in polyphosphoric acid. The dope is spun through a 36 filament spin die having an average hole diameter of about 102 μm at a rate of about 25 m/min. The dope fibers are drawn across an air gap of about 6 inch with a spin-draw ratio of about 12. The fibers are coagulated in water. Comparative Sample A is washed for 48 hours under running water, washed for 42 days in still water, and dried for 72 hours under nitrogen. Comparative Sample B is washed for 48 hours and not dried. Each sample is heat-treated at 630°C with a line tension of 3 g/denier for a time period of about 8 seconds. Those properties are set out in Table 2 Table 2

Claims: CLAIMS: 1. A process for finishing a spun and drawn dope fiber, which contains polybenzoxazole polymer and a solvent acid, having the steps of: (A) ■ coagulating the dope fiber in an aqueous coagulant; (B) washing the coagulated fiber with an aqueous washing fluid until the fiber contains no more than 8,000 ppm residual solvent acid; (C) drying the fiber; and (D) heat-treating the fiber at a temperature of at least 300°C under tension characterized in that: (1) the fiber is washed in Step (B) for less than 72 hours; and (2) the fiber is dried in Step (C) at a temperature of 120°C to 300°C until it retains no more than 3 weight percent residual moisture. A process as claimed in Claim 1 wherein the solvent acid is polyphosphoric acid. 3. A process as claimed in any of the preceding claims wherein the fiber is washed for no more than 36 hours. 4. A process as claimed in any of the preceding claims wherein the fiber is washed for no more than 12 hours. 5. A process as claimed in any of the preceding claims wherein the washed fiber contains no more than 5000 ppm residual solvent acid. 6. A process as claimed in any of the preceding claims wherein the fiber is dried until it retains no more than 1 weight percent residual moisture in step (D). 7. A process as claimed in any of the preceding claims wherein the fiber is dried in an atmosphere containing nitrogen, helium, neon or argon. 8. A process as claimed in any of the preceding claims, wherein the drying temperature is no more than 200°C until after the residual moisture content of the fiber is below 2 weight percent. 9. A process as claimed in any of the preceding claims, wherein the drying temperature reaches at least 150°C. 10. A process as claimed in any of the preceding claims wherein the polybenzazole polymer contains one or more of the repeating units:

Inventors: ALEXANDER WILLARD E, CHAU CHIEH-CHUN, FALEY TIMOTHY L, ALEXANDER, WILLARD, E., CHAU, CHIEH-CHUN, FALEY, TIMOTHY, L.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1993
Main classification: D01F6
IPC codes: D01F6, D06M11