Patent Application
20100227984
The present invention relates to synthetic fibers. More specifically, the present invention relates to a method of preparing rigid-rod polymer fibers that are resistant to hydrolysis.
Aromatic heterocyclic rigid-rod polymers are well known for their desirable mechanical properties and their thermal and thermo-oxidative stabilities. For instance, commercialized versions of poly(p-phenylene-benzobisoxazole) (PBO) fibers have been used to create high-performance materials used in such products as flame/heat-resistant fabrics, ballistic vests, balloons, satellites, sailcloth, yacht ropes, golf clubs, and as reinforcement for cement, belts, and tires.
However, it is known that PBO fibers do not maintain their physical properties over time. PBO is susceptible to degradation which reduces the mechanical performance of the fibers. As a result, the performance of the products containing the PBO fibers is also diminished. Exposure to environmental conditions such as moisture, heat, and UV radiation over time contributes to the degradation of PBO fibers. It is believed that residual acid from the manufacture of the PBO fibers contributes to the hydrolytic instability of the fibers and hastens the degradation of the fibers' performance.
Post-fabrication fiber treatments to reduce the susceptibility of PBO to degradation under adverse environmental conditions have not succeeded. For example, extraction using supercritical carbon dioxide has been attempted as a way to remove traces of phosphoric acid from PBO fibers. Extraction using supercritical carbon dioxide, followed by treatment of the PBO fibers with low molar mass base compounds (such as pyridine and morpholine) has also been attempted. However, these efforts have proven to be ineffective, time-consuming and costly.
Thus, a need exists for a method of preparing rigid-rod polymer fibers that are resistant to hydrolysis and its performance degrading effects.
In light of the foregoing, it is a first aspect of the present invention to provide hydrolysis-resistant PBO fibers.
It is another aspect of the present invention to provide a method of preparing a rigid-rod polymer fiber comprising the steps of preparing a polymer solution, extruding the polymer solution to form a filament, and exposing the filament to an aqueous base solution.
Yet another aspect of the present invention is to provide a rigid-rod polymer having a residual acid content of less than about 1.00 percent phosphoric acid content by weight.
These and other features and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying figures wherein:
Referring now to
Rigid-rod polymer fibers may be made from compositions prepared according to methods known in the art. For example, a composition to be made into rigid-rod PBO fibers may be prepared by combining selected ratios of terephthaloyl chloride, 4,6-diaminorescorcinol dihydrochloride, and an approximate 77 percent polyphosphoric acid (PPA) solution. The terephthaloyl chloride and the 4,6-diaminorescorcinol dihydrochloride can each make up from about 11 to about 21 percent of the combination. And the PPA solution can make up to about 67 to about 77 percent of the combination. The monomers are stirred in the PPA and the composition is dehydrochlorinated over a period of 24 hours under a nitrogen flow after slowly raising the reaction temperature to 105° C. to avoid foaming. The composition is cooled and a selected amount of phosphorous pentoxide (P2O5), about 26 grams, is added to provide the PPA solution with about 83 percent P2O5 content and to ensure a final polymer concentration of about 14 percent by weight in PPA. The composition is maintained and stirred at 100° C. to ensure good homogeneity and the temperature is slowly raised to 165° C. and the polymerization reaction is allowed to run for several hours. The polymerization reaction is continued at a final temperature of 180° C. for 24 hours. The resulting polymer composition, also known as “polymer dope,” may be processed into rigid-rod PBO fibers using the system 10. Although the method for preparing rigid-rod polymer fibers described below relates to PBO fibers prepared by spinning from a dope (polymer solution) of polyphosphoric acid (PPA) solution, the method may also be applied to other rigid-rod polymer fibers, such as 2,6-naphthalene PBO, that are created from raw materials in (concentrated) acidic solution.
PBO fibers may be made according to the concepts of the present invention as follows. Tank 14 holds a quantity of polymer solution 16 (PBO polymer dope in acid, prepared as described above) that is pushed by the extrusion device 12 through the spinneret 18. The opening (die) of a typical monofilament spinneret has a diameter of 20 mil (0.5 mm), though the concepts of the present invention are not limited to using a spinneret having such dimensions. The polymer solution 16 is forced through the opening in the spinneret 18 and forms an extruded PBO fiber monofilament 28. Of course, a spinneret having multiple holes could also be used, and the polymer solution 16 would be forced through the multiple holes creating several extruded PBO fiber filaments which could be combined and made into a larger PBO fiber yarn according to methods well known in the art. Given an appropriate supply of raw materials, the PBO fiber 28 may be produced continuously. In one or more embodiments, a dry-jet wet spinning technique is used with temperatures in roughly the 90-100° C. range, pressures in the roughly 1000-1200 psi range, and draw ratios as high as 40-50.
As the PBO fiber 28 is extruded from the spinneret 18, driven rollers 20 and guide rollers 22 pull the PBO fiber 28 through the first water bath 30, base bath 32, and second water bath 34 before the PBO fiber 28 is wound by the wind-up roller 24 onto the bobbin 26, forming a spool of PBO fiber. Of course, other arrangements of rollers could also be used to carry the PBO fiber through the three baths, such as one where a wind-up roller is the only driven roller and the other rollers are passive guide rollers. And, other arrangements of water and base baths could be used, such as one where the extruded PBO fibers are taken through multiple water baths before exposure to a base bath. Or, the base bath could precede any water bath. In any event, the steps discussed herein allow the removal of residual acid from the fiber during processing by exposing the extruded fiber to a neutralizing reagent (base), such as a solution of ammonium hydroxide. By treating the fibers soon or immediately after extrusion, the fibers are permeable to the neutralizing reagent. In addition to being applicable to PBO fibers, as discussed below, it is believed the present invention is equally applicable to a wide variety of polymer fibers, especially the class of “rigid-rod” polymers.
As the PBO fiber 28 is created at the spinneret 18, it passes through an air gap, then directly into the first water bath 30. The PBO fiber 28 is completely submerged in the water of the first water bath 30, and this water bath treatment washes away or dilutes any residual acid on the PBO fiber 28. In the present embodiment, the water is continuously replaced with a fresh (neutral) supply. The water bath treatment also keeps the PBO fiber 28 wet. The PBO fiber 28 is guided out of the first water bath, then into the base bath 32 and completely submerged in the basic solution contained therein. The base bath treatment neutralizes residual acid in or on the PBO fiber 28. The base bath solution is replenished and filtered as needed. Then, the PBO fiber 28 is guided into the second water bath 34 and is completely submerged in the water therein. This water bath treatment washes away residual acid, base or salt on the PBO fiber 28. As before, the water is likely continuously replaced. Finally, the PBO fiber 28 is guided out of the second water bath 34 and is wound by the wind-up roller 24 onto the bobbin 26, forming a spool of PBO fiber. In the present embodiment, the first water bath 30 and the second water bath 34 contain distilled water, and the base bath 32 contains a 5 percent aqueous ammonium hydroxide solution. Alternatively, the base bath 32 may include a solution of an alkali (proton-accepting) acid-neutralizing agent(s) other than ammonium hydroxide, preferably volatile, as long as the alkali acid-neutralizing agent is of sufficient concentration to neutralize any residual acid in the PBO fiber 28. Optionally, the spool of PBO fiber may be immersed in distilled water for a period of time (such as several days) to remove any traces of base, and then air-dried. After washing, the PBO fiber may also be heat treated to improve physical properties.
In order to demonstrate the practice of the present invention, the following examples have been prepared and tested. The examples should not, however, be viewed as limiting the scope of the invention.
A quantity of PBO fiber monofilament was prepared according to the concepts of the present invention as follows. Into a resin flask fitted with a high torque mechanical stirrer, a nitrogen inlet/outlet adapter and a side-opening for addition, was placed 12.1824 grams (g) of terephthaloyl chloride, 12.7836 g of 4,6-diaminorescorcinol dihydrochloride, and 54.54 g of a 77 percent polyphosphoric acid (PPA) solution. The monomers were stirred in the PPA and the composition was dehydrochlorinated over a period of 24 hours under a nitrogen flow after slowly raising the reaction temperature to 105° C. to avoid foaming. The composition was cooled and 26.64 g of phosphorous pentoxide (P2O5) was added to provide PPA with 83 percent P2O5 content and to ensure a final polymer concentration of 14 percent by weight in PPA. The composition was maintained and stirred at 100° C. to ensure good homogeneity and the temperature was slowly raised to 165° C. and the polymerization reaction was allowed to run for several hours. The polymerization reaction was continued at a final temperature of 180° C. for 24 hours, forming a “polymer dope.” The polymer dope was taken out of the flask for fiber spinning.
The polymer dope was transformed into PBO fibers using a system similar to that disclosed in
Various physical tests were performed on PBO fibers prepared according to the concepts of the present invention (which are referred to as “inventive PBO”). The same physical tests were performed on commercially available (prior-art) PBO fibers that had not undergone the three bath treatments (water, base, water) of the present invention. In particular, the tenacity (ultimate tensile strength per unit area) and elongation of the PBO samples were measured after periods of time in adverse environmental conditions (140° F. and 95 percent relative humidity). The median value results of these physical tests, which were normalized to account for initial differences in the number of filaments and heat treatment, are presented in Table I, and
1Inventive PBO = 85 denier monofilament
2Prior Art PBO = Toyobo Zylon High Modulus 245 denier yarn, 5.25-5.5 TPI “Z”
The results disclosed in Table I are presented graphically in
In addition, PBO fibers prepared according to the concepts of the present invention were compared to prior-art PBO fibers after periods of exposure to ultraviolet (UV) light. In particular, the PBO fibers were continually exposed to an amount of UV radiation equivalent to the amount of UV radiation in natural sunlight (created using a UVA-340 lamp at irradiance of 0.70 W/m2/nm), over a period of time, and the tenacity and elongation were measured. The median value results of these physical tests, which were normalized to account for initial differences in the number of filaments and heat treatment, are presented in Table II, and
1Inventive PBO = 85 denier monofilament
2Prior Art PBO = Toyobo Zylon High Modulus 245 denier yarn, 5.25-5.5 TPI “Z”
3Prior Art PBO = Toyobo Zylon As Spun 278 denier yarn, no twist
4Prior Art PBO = Toyobo Zylon High Modulus 545 denier yarn, no twist
The results disclosed in Table II are presented graphically in
PBO fibers prepared according to the concepts of the present invention have a residual phosphorus content of 0.090 percent as measured by elemental analysis, representing a corresponding phosphoric acid content of 0.28 percent by weight. Prior-art PBO fibers have an average residual phosphorus content of 0.38 percent as measured by elemental analysis, representing a corresponding phosphoric acid content of 1.2 percent by weight. In particular, Toyobo Zylon as-spun 278 denier yarn (Prior-Art PBO3) was found to have a residual phosphorous content of 0.39 percent, representing a corresponding phosphoric acid content of 1.2 percent by weight. Toyobo Zylon high modulus 545 denier yarn (Prior-Art PBO4) had residual phosphorous content values as high as 0.60 percent, representing a corresponding phosphoric acid content of 1.9 percent by weight. Chlorine content was below the measurable limits (0.13 percent Cl) for duplicate tests of both the inventive and prior-art samples. Thus, PBO fibers prepared according to the concepts of the present invention have less residual phosphoric acid content than prior-art PBO fibers. In other words, removing residual acid in situ during fiber processing while the fibers are still wet and permeable to the neutralizing reagent has been found to lead to a significant improvement in the final properties of the fiber.
Furthermore, treating the prior-art PBO fibers in the base, water, and base baths as disclosed above (for hours or even days) did not reduce the residual phosphorous content as measured by elemental analysis. This confirms the conclusion that removing residual acid in situ during fiber processing while the fibers are still wet and permeable to the neutralizing agent is preferable to post-fabrication treatment. Indeed, inasmuch as post-fabrication treatments using base compounds have proven to be ineffective, the improved results realized by the present invention are unexpected.
A further advantage of PBO fibers prepared according to the concepts of the present invention is that treating PBO fibers with water, base, and then water is less costly and more effective than previous methods of treating PBO fibers for prevention of hydrolysis.
Thus, it can be seen that the objects of the invention have been satisfied by the structure and its method for use presented above. While in accordance with the Patent Statutes, only the best mode and preferred embodiment have been presented and described in detail, it is to be understood that the invention is not limited thereto or thereby. Accordingly, for an appreciation of the true scope and breadth of the invention, reference should be made to the following claims.
