| PATENT ASSIGNEE'S COUNTRY | USA |
| UPDATE | 12.99 |
| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | 14.12.99 |
| PATENT TITLE |
Silver-containing poly(p-phenylene terephthalamide)/sulfonated polyaniline composite fibers |
| PATENT ABSTRACT |
Silver-containing fibers are disclosed having silver particles intermingled with domains of sulfonated polyaniline in a continuous phase of poly(p-phenylene terephthalamide). |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | 20.10.98 |
| PATENT REFERENCES CITED | This data is not available for free |
| PATENT CLAIMS |
What is claimed is: 1. A composite, silver-containing, fiber having a longitudinal axis, comprising a continuous phase of poly(p-phenylene terephthalamide) and, interspersed therein, domains of sulfonated polyaniline aligned substantially parallel with the longitudinal axis wherein the sulfonated polyaniline is present in an amount of 2 to 35 weight percent based on the combined weight of the sulfonated polyaniline and the poly(p-phenylene terephthalamide) and wherein silver particles are intermingled with the domains of sulfonated polyaniline and silver is present in an amount of 15 to 70 weight percent based on the total weight of the composite, silver-containing, fibers. 2. The composite fiber of claim 1 wherein the silver particles are interspersed throughout the entire fiber. 3. The composite fiber of claim 1 wherein the silver particles are present in a zone of impregnation at the surface of the fiber. 4. The composite fiber of claims 2 or 3 wherein there is a continuous layer of silver plated on the surface of the fiber. 5. The composite fiber of claim 1 wherein the sulfonated polyaniline and the poly(p-phenylene terephthalamide) are mutually insoluble. |
| PATENT DESCRIPTION |
BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to silver-containing, high tenacity, high modulus, electrically-conductive composite fibers of poly(p-phenylene terephthalamide) (PPD-T) and sulfonated polyaniline (SPAn). 2. Description of the Prior Art U.S. Pat. No. 5,302,415 issued on Apr. 12, 1994 discloses treatment of aramid fibers in concentrated sulfuric acid to increase adhesion of metal electrolessly plated thereon. U.S. Pat. No. 5,549,972, issued Aug. 27, 1996, discloses a process wherein silver plating is conducted on fibers of PPD-T having less than 20 weight percent water and greater than 0.5 weight percent sulfur based on the total weight of the PPD-T. U.S. Pat. No. 5,788,897, issued Aug. 4, 1998, discloses a method to prepare high tenacity, high modulus conductive fibers of PPD-T and SPAn. SUMMARY OF THE INVENTION This invention provides a composite, silver-containing, fiber having a longitudinal axis and comprising a continuous phase of poly(p-phenylene terephthalamide) and, interspersed therein, domains of sulfonated polyaniline aligned substantially parallel with the longitudinal axis wherein the sulfonated polyaniline is present in an amount of 2 to 35 weight percent based on the combined weight of the sulfonated polyaniline and the poly(p-phenylene terephthalamide) and wherein silver particles are intermingled with the domains of sulfonated polyaniline and silver is present in an amount of 15 to 70 weight percent based on the total weight of the composite, silver-containing, fibers. The silver particles can be interspersed throughout the entire fiber or they can be present in a zone of impregnation at the outer periphery of the fiber structure; and the fibers can be plated on the surface with a continuous layer of silver. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a cross sectional electron backscattering photomicrograph of a fiber of the present invention wherein plating is conducted on a PPD-T/SPAn (w/w:90/10) fiber. FIG. 2 shows the fiber of FIG. 1 at higher magnification. FIG. 3 shows a cross sectional electron backscattering photomicrograph of a fiber of the prior art wherein plating was conducted on a PPD-T fiber. DETAILED DESCRIPTION This invention provides electrically-conductive, composite fibers which exhibit high tenacity and high modulus. The primary organic components of these fibers are poly(p-phenylene terephthalamide) (PPD-T) and sulfonated polyaniline (SPAn). While the SPAn is somewhat electrically-conductive, the conductivity of the instant fibers is primarily a result of silver present in the fibers. Silver particles are present in the fibers of this invention intermingled with the SPAn. SPAn is dispersed as elongated domains aligned substantially parallel with the axis of the composite fibers and silver particles are intermingled with the domains of SPAn in that same alignment. By "poly(p-phenylene terephthalamide)" (PPD-T), is meant the homopolymer resulting from mole-for-mole polymerization of p-phenylene diamine and terephthaloyl chloride and, also, copolymers resulting from incorporation of small amounts of other diamines with the p-phenylene diamine and of small amounts of other diacid chlorides with the terephthaloyl chloride. As a general rule, other diamines and other diacid chlorides can be used in amounts up to as much as about 10 mole percent of the p-phenylene diamine or the terephthaloyl chloride, or perhaps slightly higher, provided only that other diamines and diacid chlorides have no reactive groups which interfere with the polymerization reaction. PPD-T, also, means copolymers resulting from incorporation of other aromatic diamines and other aromatic diacid chlorides, such as, for example, 2,6-naphthaloyl chloride or chloro- or dichloroterephthaloyl chloride; provided, only that the other aromatic diamines and aromatic diacid chlorides be present in amounts which permit preparation of anisotropic spin dopes. Preparation of PPD-T is described in U.S. Pat. Nos. 3,869,429; 4,308,374; and 4,698,414. Additives can be used with the PPD-T and it has been found that up to as much as 10 percent, by weight, of other polymeric material can be blended with the PPD-T or that copolymers can be used having as much as 10 percent of other diamine substituted for the diamine of the PPD-T or as much as 10 percent of other diacid chloride substituted for the diacid chloride of the PPD-T. By "sulfonated polyaniline" (SPAn), is meant polyaniline with a sulfonic acid attached to at least one repeating ring. SPAn can be represented as follows: ##STR1## -: Negative Charge +: Positive Charge .multidot.: Electron wherein "X" can be 10 to 10,000. SPAn is a "self-doped", electrically-conducting polymer, reported by Yue, Epstein and MacDiarmid in Proc. Symposium on Electroresponsive Molecular and Polymeric Systems, Brookhaven National Laboratory, October 1989, to have a conductivity of .about.0.03 S/cm without external doping. Synthesis of the material is described in J.A.C.S. 1991, Vol 113, No 7 pp 2665-2671, exhibiting a conductivity of .about.0.1 S/cm measured on pressed pellets. SPAn is not a highly conductive polymer. SPAn exhibits a conductivity of about seven orders of magnitude lower than that of metals such as silver; but the inventor herein has discovered that its unique morphology, when combined with PPD-T in the form of composite fibers, facilitates electroless plating which yields strongly adherent silver. Once silver has activated the polymer, the polymer will readily accept other metals, such as copper and nickel , for strongly adherent deposition. Up to this time, in order to have an electrolessly plated metal coating which was strongly adherent, it was believed that the PPD-T fiber surfaces had to be chemically treated to generate sites of activation for the plating process. In accordance with this invention, however, it has been discovered that the PPD-T fiber which contains SPAn does not require chemical treatment before plating. It is believed that sulfonate groups on the SPAn in elongated SPAn domains which are dispersed as described in the PPD-T, provide enduring activation sites for plating processes. This sulfonation on the SPAn is believed to serve as activation sites for deposit of silver; and, because the SPAn is dispersed throughout the fiber, those sites are present throughout the PPD-T rather than only on the fiber surface. Moreover, sulfonation sites continue as activation sites through additional processing of the PPD-T and are not deactivated by contact with water or acid or other reactive materials, as happens with plating activators of the prior art. It is believed that any amount of SPAn will improve the adhesion of silver to the composite fiber and enhance electrical conductivity; but that, below about 2 weight percent, based on the combined weight of the polymers, interconnection of SPAn domains along the fiber axis becomes much less. Higher amounts of SPAn render the composite fibers more amenable to silver adhesion for enhanced particle interconnection and electrical conductivity, but, above 40 percent, based on the combined weight of the polymers, reduction of mechanical properties and leaching of SPAn out of the composite fibers becomes excessive. For those reasons of practicality, it is preferred that the fibers of this invention should have a SPAn content of 2 to 35 weight percent, and preferably between 5 and 25 weight percent, based on combined weight of the polymers. In the practice of this invention, high tenacity, high modulus PPD-T/SPAn fibers can be made by the procedure described in U.S. Pat. No. 5,549,972, issued Aug. 4, 1998. High tenacity and high modulus are specified in the range greater than 10 gpd and 150 gpd, respectively. Fibers of this invention can be made by spinning a sulfuric acid solution of a mixture of PPD-T and SPAn into an aqueous coagulating bath to yield filaments of a coagulated combination of PPD-T and SPAn and then performing electroless plating on those filaments before all of the water has been removed. In the practice of this invention, the solution to be spun is prepared by dissolving PPD-T and polyaniline in the sulfuric acid solvent; and, during the course of dissolving, the polyaniline becomes sulfonated polyaniline (SPAn). The resulting coagulated filaments are termed "never-dried" fibers and use of such never-dried fibers is preferable in practice of this invention. Never-dried fibers may contain from 20 to as much as 120, or more, weight percent water and have never been dried to less than 20 weight percent water. While fibers which have been dried to a water content of less than 20 weight percent can also be plated in accordance with invention, such drying will have irreversibly collapsed the polymer structure of fiber, thereby severely reducing permeability thereof to activating ions and metal plating ions in subsequent processing steps. By using never-dried composite fibers in practice of this invention, plating solutions can permeate and penetrate into the fiber and come into more direct contact with more sulfonic acid sites of the SPAn in the fibers. Thus, the preferred use of never-dried fibers to expose a below-the-surface portion of the fibers to the plating solution has been discovered to yield fibers having a foundation of PPD-T with interspersed domains of SPAn throughout the fibers and silver particles intermingled with those domains. The SPAn is aligned parallel with the fiber axis and the silver particles are located along the domains. The structural characteristic is illustrated in FIG. 1 which shows an electron backscattering photomicrograph of a fiber cross-section perpendicular to the fiber axis wherein plating is conducted on a "never-dried" PPD-T/SPAn (w/w:90/10) fiber. Bright spots throughout the entire plated fiber are silver particles intermingled with SPAn domains aligned parallel with the fiber axis. FIG. 2 shows the same cross-sectional view at higher magnification where the light spots are silver particles and the edge of the fiber is shown to have a high concentration of silver particles. The silver particles are interconnected and become an integral part of the composite fibers, thereby assuring strong adhesion to the PPD-T. Silver is deposited as particles in the interior of the fiber and also, as a continuous coating or plated layer on the surface of the fiber. One of the surprising features of this invention resides in the discovery that the continuity of the coating or plated layer of silver is maintained through drying of the never-dried fibers even though the fibers may undergo substantial shrinkage during the course of drying. When composite fibers are used which have been dried to less than 20 weight percent water, permeation of the plating solutions into the fibers is greatly reduced and formation of silver particles in the interior of the fiber is limited to a zone of impregnation inside but near to the surface of the fibers. Thickness of the zone of impregnation will be controlled by several factors, including the degree of initial fiber drying and the duration of contact with the plating solution. The zone of impregnation is a peripheral volume of composite fiber having silver particles intermingled with the SPAn domains whereby the silver particles serve as anchoring points for any silver coating or plating formed at the surface of the fiber. It is estimated that 10 to 30 weight percent silver, based on the total weight of the composite fibers, is enough silver to yield interconnected silver particles and plated layers for electrical conductivity. However, for maximum electrical conductivity, it is preferred to have 30 to 70 percent silver, based on the total weight of the composite fiber. Silver content of greater than 70 percent does not appreciably increase electrical conductivity and results in inefficient use of the silver with unnecessary addition of weight. In further explanation of the operation of this invention, while the silver particles in the fibers are believed to anchor any silver coating on the fiber, sulfonate groups on the SPAn are believed to anchor the silver particles. The concentration of silver particles is a function of several factors, including degree of initial fiber dryness, duration of contact between the fibers and the plating solution concentration of the plating solution, and the like. While those factors are very important in the degree of impregnation, the rate of impregnation is somewhat self-limiting because, as the impregnation occurs, so, also, is the silver from the plating solution reduced to metallic silver, thereby, slowing further impregnation. An early step of metal plating involves activation of plating sites. For an example of a silver plating, the fibers are first immersed in an aqueous reducing agent solution such as SnCl.sub.2 /HCl. The SnCl.sub.2 -treated fibers are then contacted with a metal complex solution of silver nitrate and ammonia at a pH of 8-9.5. During immersion in the metal complex bath, imbibed stannous ions reduce silver ions to silver metal on the polymer surface to thereby activate plating sites on the polymer. Once the polymer plating sites are activated, formaldehyde is added to the metal complex solution as a reducing agent and silver ions preferentially deposit on the silver-activated plating sites. Alternatively, other metals such as, copper, nickel, cobalt, or the like can be plated on the silver-activated polymer surface with a proper reducing agent solution and metal plating solution. TEST METHODS Electrical conductivity: The electrical conductivity of fibers without silver at ambient conditions is determined by a four probe method for calculation of electrical conductivity. A fiber specimen to be tested is about 1.5 cm long. Room temperature curing silver paste is used for making four electrodes on the fiber specimen. The two inner voltage measuring electrodes are about 8 mm apart. Electrical current is applied to the two outer electrodes and the voltage corresponding to the known current is determined with a Keithley electrometer. Resistance is calculated based on Ohm's law. Conductivity in S(Siemen)/cm is calculated by normalization based on fiber cross-section and the distance between the voltage electrodes. Electrical resistance: Electrical resistance of silver-containing fiber is determined as follows: A fiber sample is attached with two pressure-contact probes. The two probes are one centimeter apart and are connected to a Keithley electrometer for determination of resistance. The resistance is reported as ohms per centimeter. Tensile test: Tenacity/Elongation/Modulus (T/E/M) of single filament at 1 inch gauge length and yarn at 5 inch gauge length are reported in grams per denier for T and M and in percent for E. The tenacity and elongation are determined according to ASTM 2101. Filament denier is determined according to ASTM D1577 using a vibroscope, and yarn denier is determined by weighing a 90 cm sample and multiplying the weight, in grams, by 10,000. Sulfur element analysis: Fiber sample is first combusted with oxygen in a flask. The generated SO.sub.2 and SO.sub.3 gases are absorbed in water. Hydrogen peroxide is added to insure that all sulfur is converted to sulfate. After boiling with platinum black to remove any excess H.sub.2 O.sub.2, the pH is adjusted to 7. The solution is then added, in a 50/50 weight ratio with isopropanol, to water. That solution is titrated with a standardized BaCl.sub.2 solution for determination of sulfate. The amount of sulfur is determined based on the sulfate concentration. Inherent viscosity: Inherent viscosity (IV) is defined by the equation: IV=ln (relative viscosity)/C where C is the concentration (0.5 gram of polymer in 100 ml 96 W. % sulfuric acid) of the polymer solution and relative viscosity is the ratio between the flow times of the polymer solution and the solvent as measured at 30.degree. C. in a capillary viscometer. |
| PATENT EXAMPLES | This data is not available for free |
| PATENT PHOTOCOPY | Available on request |
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