Electrically conductiv articles.

Background of the Invention

Polyaniline salts have been known for many years. These compounds require a polymeric support system for end-use applications. U.S. Patent 4,699,804 describes electrically conductive polymer articles.

This invention is directed to novel processes for preparing novel electrically conductive articles as well as to novel articles produced thereby.

Summary of the Invention

In accordance with the present invention, there is provided electrically conductive articles comprising organic polymeric fibers and films impregnated with polyaniline salts or having self-­adhered polyaniline salt coatings and novel processes for preparing such articles. Poly(p-phenylene terephthalamide) fiber and film are the preferred organic polymeric substrates.

Detailed Description of the Invention

The polyaniline salts useful for purposes of this invention are well-known in the art. See for example, U.S. Patent No. 4,699,804; U.S. Patent No. 4,025,691; and Mol. Cryst. Liq. Cryst. 1985, V 121, pp. 173-180. They may be prepared as disclosed in the aforementioned references. In general, they are prepared by oxidatively catalyzed polymerization of aniline in an acid medium. Typical examples of oxidizing agents for the polymerization of aniline are ammonium persulfate, potassium dichromate, ferric chloride and sodium chlorate. The amount of oxidizing agent used is sufficient to achieve the "green" emeraldine oxidation state of the polyaniline. The pH of the acidic reaction medium should be less than 4 with a preferred range of 0-1.

In accordance with the present invention, novel articles are prepared by polymerizing the aniline in aqueous solution while the solution is in contact with the fiber or film substrate. This results in the salt being self-adhered to the polymer substrate. Where it is desired to incorporate the polyaniline salt within the fiber or film substrate, an aqueous aniline solution is first absorbed within a water-swollen polymeric fiber or film, preferably of poly(p-phenylene terephthalamide) and then polymerized in situ. Ion exchange polymers in fiber or film form can also be used. These include perfluorinated polymers having pendant side chains containing sulfonic acid and/or carboxylic acid functional groups. Examples thereof are found in U.S. 4,539,084. In still another technique a polyaniline salt is incorporated in a spin dope prior to extrusion. In any case, no binder or other modification is required.

By "water-swollen polymeric fiber of film", also referred to as "never-dried polymeric fiber or film" is meant such articles which have not been dried in the course of preparation. The never-dried poly(p-phenylene terephthalamide) fibers and films generally have a water content of about 20% or more and are prepared as described in European Patent Publication No. 0,247,889, corresponding to USSN 868,667. Never-­dried polyacrylonitrile fibers are prepared as described in U.S. 3,932,571 (see column 2, lines 54-68) and are water-swollen with about 20% or more of water.

The amount of polyaniline salt to be deposited or incorporated should be sufficient to impart conductivity as would be well understood by those skilled in the art. End-uses, such as electromagnetic shielding (EMI), and microwave heating are contemplated for these products.

The following examples are illustrative of the invention except as indicated otherwise. These examples are not to be construed as limiting. They illustrate the preparation of electrically conductive articles.

Example 1

This example illustrates formation of a polyaniline salt in aqueous solution in the presence of a nonwoven fabric of poly(p-phenylene terephthalamide) and compares the result with that obtained by coating the substrate with previously formed polyaniline salt. The coated article of the invention is made by polymerizing aniline in an acid aqueous medium with an oxidizing catalyst while in contact with the article.

A piece of 8" x 8" nonwoven poly(p-phenylene terephthalamide fabric (1 ounce/square yard) was immersed in a 1N aqueous HCl solution (800 ml) containing 20 ml aniline for one hour. To the solution was then added 200 ml 1N aqueous HCl solution containing 7.5 gram ammonium persulfate. Five minutes after the addition, the solution and fabric turned green which is the color of polyaniline salt. As the polymerization proceeded, the polyaniline salt started to precipitate out. The fabric was taken out of the solution after 20 minutes of polymerization time, and washed with water several times to remove the excess HCl. The wet fabric is dried at 60°C in an oven purged with nitrogen. The dried impregnated fabric had a measured electrical resistivity of 3.2 x 10³ ohm/square at room temperature (RT).

A piece of the dried impregnated fabric was placed in a 2450 MHz waveguide (E-field = 74 volt/cm) to see whether the material was susceptible to microwave energy. The temperature of the sample rose to 150°C in 0.5 minutes. The temperature rise was attributed to resistive heating of electrically conductive polyaniline in the microwave field. This property shows that polyaniline modified materials should be useful for EMI shielding and microwave heating.

Immediately after the removal of the impregnated fabric from the solution, a fresh piece of nonwoven fabric was added to the solution. The second piece was taken out after 1 hour immersion in the solution. The polyaniline deposited on the fiber surfaces could be washed off easily with water. This control experiment shows that the fabric should be immersed in the solution before aniline polymerization occurs.

Example 2

In this example, polyaniline salt was homogeneously dispersed in poly(p-phenylene terephthal­amide) fiber. The dispersion was accomplished by incorporating polyaniline in base form in the spin dope and extruding it into fiber. The spin dope, 322.4 g 100% sulfuric acid and 77.6 g of poly(p-phenylene terephthalamide) having an inherent viscosity of 6.5, was prepared by a standard procedure. To the spin dope was added 0.8 g polyaniline dissolved in 40 g 100% sulfuric acid. The mixture was stirred for about 2 hours to ensure homogeneity. The polyaniline used in the spin dope was prepared as follows. An aqueous aniline solution consisting of 40 ml aniline, 100 ml 35% HCl solution and 900 ml water was mixed with 23 g ammonium persulfate. The polyaniline precipitate collected was in the emeraldine salt form. The poly­aniline was then washed with ammonium hydroxide. The process converted the emeraldine salt, green in color, to the base form, which is blue. The base form of polyaniline is used for spin dope preparation. The spin dope containing 17.6% poly(p-phenylene terephthalamide) was spun through an air gap into a coagulation bath as described in U.S. Patent No. 3,767,756 and the fibers on a bobbin were thoroughly washed with water. The fibers thus obtained are green and the polyaniline is dispersed homogeneously throughout as observed from optical photographs of the fiber cross-section. The yarn (20 filaments, 39 denier) tenacity is 22 gpd.

Some of the green fiber was placed in a 2450 MHz waveguide (E-Field = 371 volt/cm) to see whether it absorbed microwave energy. The temperature of the sample rose to 148°C. The temperature rise is attributed to resistive heating of the conductive polyaniline.

Example 3

This example illustrates the in-situ formation of a polyaniline salt while in contact with a yarn of poly(p-phenylene terephthalamide).

A 40-yard sample of dried as-spun poly(p-­phenylene terephthalamide) (200 filaments, 379 denier) was wound up on a glass rack. The assembly was immersed in a beaker containing 1,650 ml water, 150 ml of 35% HCl solution and 28 ml aniline. To the solution 7.3 g of ammonium persulfate dissolved in 30 ml of 1N HCl was added. The reaction was allowed to proceed for 2 minutes after the solution turned greenish. The polyaniline-modified fiber was then washed with water to remove the excess HCl. The resulting fiber was green in color. The dried modified fiber was comparable to an as-spun control in tenacity/elonga­tion/modulus T/E/M (25.6 gpd/3.1%/790 gpd vs. 25.8/3.2/­762). Based on denier data, the polyaniline salt picked up by the fibers constituted 1.4% by weight. The electrical resistance of the modified yarn is one megaohm per centimeter at RT. The fiber without polyaniline has a much higher resistance (at least 6 orders of magnitude higher).

Example 4

In this example, a never-dried poly(p-­phenylene terephthalamide) yarn was modified with a polyaniline salt by an imbibition process. The never-dried yarn was first imbibed with aniline before being immersed in an oxidizing solution to polymerize the aniline and form the salt of the polymer. The never-dried yarn (20g) was pretreated in a solution of 200 ml water, 20 ml aniline and 15 ml of an alkyl (C₁₂-C₁₆) trimethyl ammonium halide solution (active ingredient, 20%). The mixture was brought to a boil. About half of the above-treated yarn was immersed in a solution containing 700 ml water, and 10 ml of a solution formed from 20 g potassium dichromate and 500 ml sulfuric acid. The solution was brought to a boil. The fibers were then rinsed with water before being boiled in 10% phosphoric acid solution and oven dried. The electrical resistivity of the resulting green fibers was 1.1 x 10² ohm-cm at RT, measured as follows. The modified fibers, confined in an insulating cylinder, were placed between two stainless steel electrodes. The electrical resistance of the fibers was measured while pressure was applied on the two electrodes assuring good contact. The never-dried yarn of this Example 4 may be replaced by never-dried polyacrylonitrile yarn to obtain a conductive product.

Example 5

Never-dried, polyacrylonitrile fibers were first imbibed with aniline according to the procedure described in Example 4. About one gram of the aniline-­treated fibers were immersed in a 100 ml 1N HCl aqueous solution containing 3.36g ammonium persulfate and 1.5 ml aniline. The polymerization reaction proceeded with stirring for two hours. The resulting green fibers, impregnated with polyaniline salt, were then dried in a vacuum oven. This was followed by a thorough rinsing with water (three times). The washed fibers were dried again before measuring the electrical resistance. The electrical resistivity of the fibers at RT was 9.5 x 10² ohm-cm as measured by the method of Example 4.

Example 6

A piece of 4" x 4" fabric of poly(ethylene terephthalate) was first scoured in 250 ml water plus 2.5 ml "Merpol" HCS, a wetting agent, at 70°C for one hour. The scoured fabric was then soaked in 300 ml water containing 15 ml aniline at 60°C for one hour. The aniline soaked fabric was then placed in a 1 N aqueous HCl solution (300 ml) containing 5 ml aniline. To the solution 7 g of ammonium persulfate was added. After 30 minutes, the fabric was taken out of the solution and washed with water to remove the excess HCl. The resulting fabric was green in color and electrically conductive (surface resistivity = 840 ohm/square).

A piece of fabric modified with polyaniline as above was placed in a 2,450 MHz waveguide (E-field = 74 volt/cm). The sample reached 160°C in 0.5 minutes. This property shows that the polyaniline modified material is useful for EMI shielding and microwave heating.

Example 7

A 7 ml thick film of "Nafion" 117, Du Pont's trademark for perfluorinated cationic exchange polymer with pendant side chains containing sulfonic acid functional groups (equivalent weight of 1100) was pressed tightly between 2 flanges on the connecting side arms from 2 glass containers. One contained 1 N aqueous HCl (200 ml)/10 ml aniline solution. The other contained 200 ml water plus 3.22 g ferric chloride. After 1 hour, the film turned green and became dark very quickly. The reaction was allowed to proceed for 16 hours. The green color is attributed to the emeraldine salt form of polyaniline formed from the oxidation of aniline within the film. The side in contact with the ferric chloride solution was electrically conductive due to the presence of polyaniline. The polyaniline was also incorporated in the body of the film beneath the surface (∼60 microns deep). Electrical resistivity of the polyaniline impregnated film is 100 ohm-cm.

A piece of polyaniline modified film prepared in the same way as above was placed in a 2,450 MHz waveguide (E-field = 74 volt/cm). The temperature of the sample rose to 180°C in 1 minute and reached an equilibrium temperature of 200°C. This unique property shows that the material is useful for EMI shielding and microwave heating.