WO1994012700A1

WO1994012700A1 - Low denier polybenzazole fibers and the preparation thereof

Info

Publication number: WO1994012700A1
Application number: PCT/US1993/011587
Authority: WO
Inventors: Chieh-Chun Chau; Myrna Serrano
Original assignee: The Dow Chemical Company
Priority date: 1992-12-03
Filing date: 1993-11-30
Publication date: 1994-06-09
Also published as: IL107735A0; TW257798B; AU5682594A; JP3265579B2; CN1098148A; KR950704548A; JPH08504007A; CN1051341C; MX9307665A

Abstract

Low denier polybenzazole fibers are described and claimed. Also claimed is a process for making these low denier polybenzazole fibers. This process is spinning the fibers out of a low concentration spinning dope through small orifices (.0051 cm to .018 cm) at a spin draw ratio of from 12 to 60 and then winding without a break at least 750 kilometers of this fiber.

Description

LOW DENIER POLYBENZAZOLE FIBERS

AND THE PREPARATION THEREOF

This invention relates to polybenzazole ("PBZ") fibers and processes for making them.

Polybenzazole fibers include polybenzothiazole ("PBT") fibers or polybenzoxazole ("PBO") fibers. These PBZ fibers are high performance fibers which possess good chemical resistance, and thermal stability and mechanical properties. These features make them an excellent choice of material for high performance fiber products and composite applications.

Spinning polybenzazole fibers from solutions of PBZ in various solvents (see

Encyclopedia of Polymer Science and Technology, Vol . 1 1 , pp. 601-635) yields fibers with a nominal diameter of from 16 to 20 microns It is desirable to make PBZ fiber smaller than 12 microns in diameter in order for the fiber to be useful in certain end-use applications.

Heretofore, it has been unknown how to uniformly spin PBZ fiber with a diameter lower than 12 microns.

Another common way of describing low diameter fibers is to refer to them as low denier fibers. Denier is the weight in grams of 9000 meters of any fiber. Denier is a direct numbering system in which the lower numbers represent the finer sizes and the higher numbers the coarser sizes. As denier has the units of weight per length and diameter is a unit of length, denier and diameter are not interchangeable units when comparing direct numbers. The equation for their relationship for polybenzoxazole fiber with a density of 1 .58 is as follows:

diameter of fiber (in microns) = (square root of the denier/filament) x 9.4625.

The terms denier and diameter are, however, suitable for interchangeability when used colloquially to refer to small diameter, very fine fibers.

One aspect of the present invention is a process for making low diameter polybenzazole fiber comprising:

(1) spinning a lyotropic liquid-crystalline polybenzazole dope that contains polybenzazole polymer and a sol vent through the orifices of a spinneret, wherein the hole diameter of said orifices is .018 centimeters or less, to create a dope fiber;

(2) drawing said dope fiber across an air gap of 40.6 centimeters or less, using a spin draw ratio of between 12 and 60; and

(3) removing a major part of the sol vent from the fiber by contacting it with a non-solvent; and

(4) winding at least 750 km of unbroken fiber.

A second aspect of the present invention is a polybenzazole fiber with a diameter of between 4 microns and 12 microns. Polybenzazole is a rigid rod polymer as described in the Encyclopedia of Polymer Science and Technology in Vol. 1 1 at pp. 601-635.

The present invention uses fibers containing polybenzoxazole or

polybenzothiazole polymers. Polybenzoxazole, polybenzothiazole and random, sequential and block copolymers of polybenzoxazole and polybenzothiazole are described in references such as Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Patent 4,703,103 (October 27, 1987): Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Patent 4,533,692 (August 6, 1985); Wolfe et al., Liquid Crystalline Poly(2, 6- -Benzothiazole) Compositions, Process and Products, U.S . Patent 4,533,724 (August 6, 1985); Wolfe, Liquid Crystalline Polymer Compositions. Process and Products, U.S Patent 4,533,693 (August 6, (1985); Evers, Thermooxidatively Staple Articulated p-Benzobisoxazoie and p-Benzobisthiazole Polymers, U.S. Patent 4,359,567 (November 16, 1982); Tsai et al ., Method for Making Heterocyclic Block Copolymer, U.S. Patent 4,578,432 (March 25, 1986); 1 1 Ency. Poly. Sci. & Eng. , Polybenzothiazoles and Polybenzoxazoles, 601 (J. Wiley & Sons 1988) and W. W. Adams et al., The Materials Science and Engineering of Rigid-Rod Polymers (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)

wherein:

Each Ar represents an aromatic group. The aromatic group may be heterocycli c, such as a pyridinyiene group, but it is preferably carbocyclic. The aromatic group may be a fused or unfused polycyclic system, but is preferably a single six-membered rin g. Size is not critical, but the aromatic group preferably contains no more than about 18 carbon atoms, more preferably no more than about 12 carbon atoms and most preferably no more than about 6 carbon atoms. Examples of suitable aromatic groups include phenylene moieties, tolylene moieties, biphenylene moieties and bisphenylene ether moieties. Ar¹ in AA/BB-mer units is preferably a 1 ,2,4,5-phenylene moiety or an analog thereof. Ar in AB-mer units is preferably a 1,3,4-phenylene moiety or an analog thereof.

Each Z is independently an oxygen or a sulfur atom.

Each DM is independently a bond or a divalent organic moiety that does not interfere with the synthesis, fabrication or use of the polymer. The divalent organic moiety may contain an aliphatic group, which preferably has no more than about 12 carbon atoms, but the divalent organic moiety is preferably an aromatic group (Ar) as previously described. It is most preferably a 1 ,4-phenylene moiety or an analog thereof.

The nitrogen atom and the Z moiety in each azole ring are bonded to adjacent carbon atoms in the aromatic group, such that a five-membered azole ring fused with the aromatic group is formed.

The azole rings in AA/BB-mer units may be in cis- or trans-position with respect to each other, as illustrated in 1 1 Ency. Poly. Sci. & Eng., supra, at 602.

The polymer preferably consists essentially of either AB-polybenzazole mer units or AA/BB-polybenzazole mer units, and more preferably consists essentially of AA/BB- -polybenzazole mer units. The molecular structure of the polybenzazole polymer may be rigid rod, semi-rigid rod or flexible coil. It is preferably rigid rod in the case of an AA/BB- -polybenzazole polymer or semi-rigid in the case of an AB-polybenzazole polymer. Azole rings within the polymer are preferably oxazole rings (Z = 0). Preferred mer units are illustrated in Formulae 2(a)-(h).

cis-polybenzoxazole

Poly[benzo(1,2-d:5,4-d')bisoxazole-2,6-diyl-1,4-phenylene]

trans-polybenzoxazole

Poly[benzo(1,2-d:4,5-d')bisoxazole-2,6-diyl-1,4-phenylene]

trans-polybenzothiazole

cis-polybenzothiazole

AB-PBO

Poly(2,5-benzoxazole)

AB-PBO

Poly ( 2 , 6-benzoxazole )

Poly ( 2 , 5-benzothiazole )

Poly ( 2 , 6-benzothiazole ) The polymer more preferably consists essentially of mer units selected from those illustrated in 2(a)-(h), and most preferably consists essentially of a number of identical units selected from those illustrated in 2(a)-(c).

Each polymer preferably contains on average at least about 25 mer units, more preferably at least about 50 mer units and most preferably at least about 100 mer units. The inherent viscosity of rigid AA/BB-polybenzazole polymers in methanesulfonic acid at 25°C is preferably at least about 10 deciliters/gram ("dL/g"), more preferably at least about 15 dL/g, and most preferably at least about 20 dL/g. For some purposes, an inherent viscosity of at least about 25 dL/g or 30 dL/g may be best. Inherent viscosity of 60 dL/g or higher is possible, but the inherent viscosity is preferably no more than about 45 dL/g. The inherent viscosity of semi-rigid AB-polybenzazole polymers is preferably at least about 5 dL/g, more preferably at least about 10 dL/g and most preferably at least about 15 dL/g.

The polymer is fabricated into fibers and films by spinning or extruding from a dope. If freshly made polymer or copolymer is not available for spinning or extruding, then previously made polymer or copolymer can be dissolved in a solvent to form a solution or dope. Some polybenzoxazole and polybenzothiazole polymers are soluble in cresol, but the solvent is preferably an acid capable of dissolving the polymer. The acid is preferably nonoxidizing. Examples of suitable acids include polyphosphoric acid, methanesulfonic acid and sulfuric acid and mixtures of those acids. The acid is preferably polyphosphoric acid and/or methanesulfonic acid, and is more preferably polyphosphoric acid.

The dope should contain a high enough concentration of polymer for the polymer to coagulate to form a solid article but not such a high concentration that the viscosity of the dope is unmanageable to handle. When the polymer is rigid or semi-rigid, then the concentration of polymer in the dope is preferably high enough to provide a liquid crystalline dope. The concentration of the polymer is preferably at least about 7 weight percent, more preferably at least about 10 weight percent and most preferably at least about 14 weight percent. The maximum concentration is limited primarily by practical factors, such as polymer solubility and, as already described, dope viscosity. Because of these limiting factors, the concentration of polymer is seldom more than 30 weight percent, and usually no more than about 20 weight percent.

Suitable polymers or copolymers and dopes can be synthesized by known procedures, such as those described in Wolfe et al., U.S. Patent 4,533,693

(August 6, 1985); Sybert et al., U.S. Patent 4,772,678 (September 20, 1988); Harris, U.S. Patent 4,847,350 (July 1 1, 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 Polymers at 253-64 (Materials Res. Soc. 1989). In summary, suitable monomers (AA-monomers and BB-monomers or AB-monomers) are reacted in a solution of nonoxidizing and dehydrating acid under nonoxidizing atmosphere with vigorous mixing and high shear at a temperature that is increased in step-wise or ramped fashion from a starting temperature of no more than about 120°C to a final temperature of at least about 190°C. Examples of suitable AA-monomers include terephthalic acid and analogs thereof. Examples of suitable BB-monomers include 4,6-diaminoresorcinol, 2,5-diaminohydroquinone, 2,5-diamino-1 ,4-dithiobenzene and analogs thereof, typically stored as acid salts. Examples of suitable AB-monomers include 3-amino-4- -hydroxybenzoic acid, 3-hydroxy-4-aminobenzoic acid, 3-amino-4-thiobenzoic acid, 3-thio-4- -aminobenzoic acid and analogs thereof, typically stored as acid salts.

Spinning of PBO Fiber

Preparation of PBO "Dope"

A PBO dope is a solution of PBO polymer in a solvent. Polybenzoxazole is only soluble in very highly protic acid solvents such as methanesulfonic acid or polyphosphoric acid. A preferred solvent is polyphosphoric acid ("PPA"). The preferred concentration of PBO in the polyphosphoric acid is approximately 14 weight percent. The intrinsic viscosity of the PBO/PPA polymer dope should be in the range of 22 to 45 dL/g (based on measuring in a methanesulfonic acid solution at 25°C and a .05 g/dL concentration). The fiber may be spun from a monofilament or multifilament line. An example of a useful monofi lament fiber spinning line is shown on page 625 of a review article by J. F. Wolfe (see "Polybenzothiazoles and Polybenzoxazoles," Encyclopedia of Polymer Science and Engineering, 2nd ed., Vol. 1 1 , pp. 601-635). The spinning equipment preferably contains a spinneret having one or more orifices and a means to impel the dope through the orifices. When the spinneret contains multiple orifices, the equipment preferably further contains a spin die to bring the dope to each orifice with about the same pressure and flow rate. The means to impel the dope may be, for instance, a pump, a piston or a single- or multiple-screw extruder. The spinneret and spin die together are known as the spin block. Each part of the spin block must be capable of handling highly viscous, corrosive polymer dope solutions.

Spinning a Dope Fiber

Once a dope has been added to the spin block it is heated until it reaches the spinning temperature. The spinning temperature can range from between 135°C to 180°C with a preferred spinning temperature range between 140 and 170°C and the most preferred spinning temperature range being 150°C to 160°C.

The polymer dope is spun through orifice holes ranging in size from .0051 to .018 centimeters (2 to 7 mils) with a preferred range of from .0076 to .015 centimeters (3 to 6 mils), and with the most preferred diameter .013 centimeters.

The shear rate of the dope as it exits the spin block is in the range of between 300 and 4000 reciprocal seconds, a more preferred range for the shear rate is between 500 and 3000 reciprocals seconds with the most preferred shear rate being 1500 reciprocal seconds.

As the PBO polymer dope is spun through the spinneret, dope fibers are formed. These dope fibers exit the spin block and enter a space known as an "air gap" which exists between the exit side of the spin block and a coagulation bath. The gas in the "air gap" may be air, but it may also be another gas such as nitrogen, carbon dioxide, helium or argon. The temperature in the air gap is preferably between about 0°C and 150°C, more preferably between about 0°C and 100°C and most preferably between 50°C and 100°C. The air gap is in the range of .64 to 41 centimeters (.25 to 16 inches), with a more preferred range being between 1.3 to 15 centimeters (0.5 to 6 inches) and the most preferred range being 2.5 to 10 centimeters (1 to 4 inches).

The spin-draw ratio is the ratio of the take-up velocity of the fiber divided by the extrusion velocity of the dope. The extrusion velocity is either determined by the dope volumetric flow rate in the extruder, or by the free-drop velocity of the spun dope out of the spinneret. For this work, the free drop velocity of the spun dope out of the spinneret was used to determine the spin draw ratio. The spin-draw ratio forthis spinning was kept between 12 and 60 with a preferred range being from 13 to 25 and the most preferred spin-draw ratio being about 15. Once the dope fibers leave the air gap and enters the coagulation/washing bath or encounters a coagulation/washing spray, the solvent (polyphosphoric acid) remaining in the fiber begins phase-separating from the PBO polymer in the fiber and diffusing into the coagulant. This phase separation process is known as coagulation. Any coagulation or washing bath/spray used can contain water or water/acid mixtures, with the preferred acid being phosphoric acid at a concentration of 30 percent or less. Other coagulants or washing liquids forthe fiber can include organic solvents such as acetone, methanol or acetonitrile Washing, Drying and Heat Setting the Fiber

After the fiber is washed it is dried and then heat set. The fibers are first dried to remove the residual surface water. Once the fibers are dry they are heat-treated. The heat treatment takes place preferably through a furnace that contains an inert gas such as nitrogen Tension is placed on the fiber as it goes through the heat treating element. Heat treating can take place at any temperature between 300°C and 650°C wιth a preferred temperature range of between 400°C and 650°C and the most preferred temperature being 600°C. The residence time for heat treating depends upon the temperature, with the hotter the temperature the less time required. A range of time for heat treatment is between one second and one minute. A preferred length of time is between 3 seconds and 45 seconds, the more preferred range of time for heat treating is between 5 to 20 seconds and the most preferred range of time is between 8 to 15 seconds. During the heat treatment the fibers are under tension.

When following the processing conditions of this invention it is possible to spin fiber for great lengths of time without experiencing a break in the fiber due to instability of the spinning process. The measurement for length of time the spinning process can be run without experiencing a break is expressed in kilometers of unbroken fiber collected. For this invention the amount of unbroken fiber that can be collected is at least about 500 kilometers, more preferably at least about 600 kilometers and most preferably at least about 800 kilometers.

The physical properties of the low denier fibers produced by this inventive process have been measured. It was found that the low denier fiber made by this method had a heat treated tensile strength that ranged from between 4.31 GPa to 6.07 GPa (625 ksi to 880 ksi), while the as-spun tensile modulus of the instant fibers range from between 172.4 GPa to 310.2 GPa (25 msι to 45 msi).

These polybenzoxazole low denier fibers are useful wherever high performance solvent resistant, cut resistant, flame resistant, fibers are useful Furthermore the tensile and modulus properties make them of value in composite applications.

The following examples are given as specific illustrations of the invention It should be understood, however, that the invention is not limited to the specifics set forth in the examples. Example 1 - Monofilament Spinning

Fourteen weight percent PBO/PPA dopes with an intrinsic viscosity (" IV") in the range of 22 to 40 dL/g (determined based on methane sulfonic acid solutions, 25°C, 0 05 g/per dL) were prepared by polymerizing PBO in PPA. These polymer dopes were then used for fiber spinning. Fiber spinning was performed by using a spin block with a 4-hole capability. For each spinning run the diameter of the hole or "orifice" (its more technical designation) the fibers were spun through was specified . Fibers were spun at a barrel temperature of 150°C . The dope fibers exited from the spinneret and travel across 20 mm of air gap and were then collected on the winding spool immersed in 15°C water at various winding speeds. Fibers were washed in running water for 48 hours, air dried for 24 hours and vacuum dried for 4 hours. They were then heat-treated to 450°C under tension. The spinning conditions and fiber diameters are shown in the following table. Samples 1 -7 were prepared from PBO dope with an IV = 19 dL/g.

*IV of this PBO dope is 26 dL/g Example 2 - Spinning at Low Diameter Orifice Size

Fourteen weight percent PBO/PPA dopes with an IV in the range of 22 to 40 dL/g

(determined based on methanesulfonic acid solution, 25°C, 0.05 dL/g concentration) were prepared by using the same method as that in Example 1.

Fiber spinning was performed by using the same apparatus as described in

Example 1 , with a 4 hole spinneret. Dopes are handled the same way as that in Example 1.

Fibers were spun at a barrel temperature of 150°C and the dope fibers exiting from the spinneret travel across a 20 mm air gap to be collected on a winding spool immersed in 150 water at various winding speeds. Fibers were washed in running water for 48 hours, air dried for 24 hours and vacuum dried for four hours. They were then heat treated at 450°C under tension.

Single fibers of 5.1 centimeters (two inch) gauge length are stretched in an

Instron™ Tensile Tester at a strain rate of 0.01/min. Test results are reports in ksi which stands for "thousand pounds per square inch" or msi which stands for "million pounds per sguare inch." 1 ksi = 6.895 Mega Pascals (abbreviated MPa), 1 msi = 6.895 Giga Pascals (GPa). Results of the fiber spinning and tensile strength and modulus tests are shown in Table 2.

*average of 12 to 19 samples are used for the heat treated tensile strength and heat treated tensile modulus numbers

Example 3 - Multifilament Spinning of Low Denier PBO Fiber

PBO dopes were prepared by polymerizing PBO in PPA. The dopes, about 1000 g, were used for fiber spinning by using a multifilament fiber spinning apparatus. The multifilament fiber spinning apparatus included a mixing device which mixed and

homogenized the dope under vacuum, and a spin block which received, filters and delivered the dope to a 36 hole (orifice) spinning die. A water tank which contained 2 or more free rotating wheels arranged in series was placed underneath the spinning die to receive the spin dopes and to coagulate the dope fibers in water. PBO dopes were loaded in the spinning apparatus and mixed for at least 4 hours and then delivered to the spin block. The dopes were then spun out of the spinning die at 160°C. The size of the orifices in the spin block was approximately .01 centimers (4 mils). The exiting dope fibers passed across about a 7.62 centimeters (three inch) air gap and were immersed in water and guided by the wheels across the distance of about 91.4 centimeters (3 feet) and then drawn by a fiber winding godet. The fibers were then delivered to a winder.

The as-spun fibers were then washed in running water for 48 hours and then stored and dried in a nitrogen-purged chamber. Some of the fibers tested were heat treated under tension. Both the as spun and dried PBO fibers were used for microscopic observations and mechanical tests. Tensile tests were performed with a 25.4 centimeter (10 inch) gauge length and a yarn twist factor equal to 6. Test results are reports in ksi which stands for "thousand pounds per square inch" or msi which stands for "million pounds per square inch. " 1 ksi = 6.895 Mega Pascals (abbreviated MPa), 1 msi = 6.895 Giga Pascals (GPa). The test results are shown in Table 3

*Fiber was heat treated at 500°C,

**Fiber was heat treated at 630° C,

xSamples 19 and 19a are not examples of this invention

The examples show that the tensile properties are much enhanced for fibers of 10 micron diameter or smaller spun from a .01 centimeters (4 mil) hole size die as compared with fibers prepared from a .05 (20 mil) hole size die. The small diameter fibers possess 50 percent more tensile modulus and 20 percent more tensile strength in the as spun state.

Claims

CLAIMS :

1. A process for preparing low diameter polybenzazole fiber having the steps of: (a) spinning a lyotropic liquid-crystalline polybenzazole dope that contains polybenzazole polymer and a solvent through the orifices of a spinneret, wherein the hole diameter of said orifices is .018 centimeters or less, to create a dope fiber;

(b) drawing said dope fiber across an air gap of 40.6 centimeters or less, using a spin-draw ratio of between 12 and 60; and

(c) removing a major part of the solvent from the fiber by contacting it with a non-solvent; and

(d) winding at least 750 km of unbroken fiber.

2. The process of Claim 1 which also includes the steps of washing, drying and heat-treating the fiber.

3. The process of Claim 1 wherein the hole diameter of said orifices is greater than .0051 centimeters and less than .018 centimeters.

4. The process of Claim 1 in which the low diameter polybenzazole fiber is polybenzoxazole fiber.

5. A polybenzazole fiber with a diameter between 4 and 12 microns.

6. The polybenzazole fiber of Claim 4 in which the polybenzazole is polybenzoxazole.

7. The polybenzazole fiber of Claim 4 in which the polybenzazole is polybenzothiazole.

8. The polybenzazole fiber of Claim 4 in which there is an unbroken length of at least 500 kilometers of fiber.

9. The polybenzazole fiber of Claim 4 in which there is an unbroken length of at least 600 kilometers of fiber.

10. The polybenzazole fiber of Claim 4 in which there is an unbroken length of at least 750 kilometers of fiber.