MX2007014659A - Electroconductive aramid paper. - Google Patents

Abstract

This invention relates to aramid papers, and a process for making such papers, the papers comprising 5 to 65 parts by weight aramid fiber, 30-90 parts by weight aramid fibrids, and 1-20 parts by weight of conductive filler, based on the total weight of the aramid fiber, fibrids, and filler; the papers having an apparent density of not more than 0.43 g/cm³ and a tensile index not less than 60 Nm/g.

Description

PAPER OF ARAMIDA ELECTROCONDUCTOR FIELD OF THE INVENTION This invention relates to a suitable electroconductive aramid paper for the interference of electrostatic discharge and / or protection against electromagnetic interference.

BACKGROUND OF THE INVENTION AOMIDE paper with NOMEX® Conductor Carbon Blend Type 843 consists of short fibers and microscopic fibrous particles of the NOMEX® brand mixed with carbon conductive fibers. This paper has been available in both calendered and hot versions without calendering. The uncalendered version of this paper has a basis weight of approximately 40 g / m2, a density of approximately 0.29 g / cm3, and a tensile strength of approximately 16 N / cm, which corresponds to the tension index of 40 * m / g, and can be easily saturated with polymer resins. However, it has been found that this paper does not have adequate tensile strength for automated ribbon winding of the conductors, causing the breaking and tearing of the aramid ribbons when wound up since it uses the lowest of the tensions normally used by automatic winding devices. The hot-calendered Ref. 187400 version of this paper has an improved tensile strength of approximately 35 N / cm (a tension rating of approximately 90 N * m / g) and is strong enough for automatic tape winding; however, this calendered paper is less saturable and less mouldable, because after calendering the resulting paper is denser (approximately 0.64 g / cm3). Paper saturation is important for the paper used as an electrical insulator because in different applications the insulation is wound around a part, and the rolled part is then impregnated with a polymer resin to eliminate substantially any air space in the winding and to reduce the non-uniformity of the electric field and the subsequent premature failure of the insulation. After the paper is wound around a coiled portion, the paper must be sufficiently porous to allow the polymer resin to pass through the paper to completely impregnate both the paper and any other coils that may be present. It is also desired that the conductive paper has a certain level of surface resistivity to avoid the accumulation of charge and provide optimum electrical protection in the particular application. Thus, a preferable surface resistivity of the conductive tapes for the outer layers of the main wall insulation of the coils in high voltage motor stators is in the range of approximately 15.50 to 62 ohms / cm2 (100 to 400 ohms / in2) . It is also very important to have a manufacturing process that allows good control of the surface resistivity of the final paper. The surface resistivity of NOMEX® type 843 lightweight hot calendered paper (approximately 108.5 ohms / cm2 (700 ohms / in2) in the machine direction and approximately 279 ohms / cm2 (1800 ohms / in2)) in the transverse direction) is approximately seven times that of uncalendered paper (14.7 ohms / cm2 (95 ohms / in2) in the machine direction and approximately 38.7 ohms / cm2 (250 ohms / in2) in the transverse direction). The U.S. patent No. 2,999,788 to Morgan; 3,756,908 of Gross; and 4,481,060 to Hayes discloses papers based on microscopic fibrous particles from synthetic polymers including microscopic fibrous particle papers (aramid) and their combinations with different fibers. The U.S. Patent No. 5,233,094 to Kirayoglu et al., Discloses a process for making resistant paper comprising 45-97 wt.% P-aramid fiber, 3-30 wt.% Microscopic fibrous particles of m-aramid and 0-30 % by weight of quartz fiber. The paper is produced by forming, calendering and high temperature heat treatment at least 266 ° C (510 ° F).

The U.S. Patent do not. 5,12,012 of Henderson et al., Reveals high strength aramid paper from short fibers and microscopic fibrous particles, and carbon fibers are among the possible types of short fibers. The necessary mechanical properties are achieved after hot pressing of the paper in the press at a temperature of 279 ° C. The U.S. patent No. 5,316,839 to Kato et al., Discloses multilayer aramid paper with conductive fibers of the structure. The paper is prepared by forming by hot compression or hot calendering at above the vitreous transition temperature of polymethaphenylene isophthalamide (275 ° C). Previously, aramid papers with conductive fillers required hot calendering or hot compression to make the paper more resistant and through this it is suitable for the automated winding of the tape. At the same time, calendering or hot compression significantly changes the electrical properties, as well as reducing its free volume and capacity to be saturated and impregnated by a resin. Therefore what is necessary is a conductive aramid paper having the desired electrical properties, be saturable by the resins, and also strong enough to be processed in automated tape winding machines.

BRIEF DESCRIPTION OF THE INVENTION The invention relates to aramid paper comprising from 5 to 65 parts by weight of aramid fiber, 30-39 parts by weight of microscopic fibrous particles of aramid and 1-20 parts by weight of the conductive filler, Based on the total weight of the aramid fiber, microscopic fibrous particles, the paper has a bulk density of no greater than 0.43 g / cm3 and a tensile index of not less than 60 Nm / g. The invention is also directed to processes for making aramid paper comprising the steps of forming an aqueous dispersion of 5 to 65 parts by weight of the aramid fiber, 30-90 parts by weight of the microscopic fibrous particles of aramid, and -20 parts by weight of the conductive filler, based on the total weight of the aramid fiber, microscopic fibrous particles, and filler; mixing the dispersion to form a slurry; draining the aqueous liquid from the slurry to produce a wet paper composition; drying the wet paper composition; and heat treating the paper at or above the glass transition temperature of the polymer in the polymeric aramid fibrous particles without paper consolidation.

DETAILED DESCRIPTION OF THE INVENTION This invention relates to aramid paper comprising 5 to 65 parts by weight of the aramid fiber, 30-90 parts by weight of the microscopic fibrous particles of aramid, and 1-20 parts by weight of the filling conductor, based on the total weight of the aramid fiber, microscopic fibrous particles, and filler, the paper has a bulk density no greater than 0.43 g / cm3 and a tensile index no greater than 60 Nm / g. Surprisingly, the inventors have found that a resistant paper does not change significantly in the free volume of paper or the surface resistivity can be made by heat treating the formed paper at a temperature of approximately above the glass transition temperature of the aramid polymer of the microscopic fibrous particles but without applying substantial pressure to the sheet in the hot state to consolidate or compress the paper. The papers of this invention include fibers and microscopic fibrous particles made of aramid polymers. Aramid polymers are polyamides wherein at least 85% of the aramid bonds (-CO-NH-) are directly attached to two aromatic rings. The additives can be used with the aramid and it has been found that up to as much as 10 percent, by weight, of another polymeric material can be mixed with the aramid. The copolymers can be used having as much as 10 percent of other diamines substituted by the diamine of the aramid or as much as 10 percent of other substituted diacid chloride for the diacid chloride of the aramid. Methods for making the aramid polymers and fibers are disclosed in U.S. Patent Nos. 3,063,966; 3,133,138; 3,287,324; 3,767,756 and 3,869,430. In some preferred embodiments of this invention, the aramid polymers are meta- and para-oriented aramides, with poly (meta-phenylene isophthalamide) and poly (para-phenylene terephthalamide), with the aramide polymers being preferred. The papers of this invention comprise aramid fibers. In different embodiments of the invention, the aramid fiber can be in the form of short fiber or pulp. By "short fiber" means fibers having a length of about 2 to 25 millimeters, preferably 3 to 7 millimeters; the fibers preferably have a diameter of about 3 to 20 microns, preferably 5 to 14 microns. If the length of the short fibers is less than about 2 millimeters it is difficult to make resistant papers and if the length is greater than about 25 millimeters, it is difficult to form a uniform fabric by a wet spinning method. If the short fiber diameter is less than about 3 microns, it may be difficult to produce it with adequate uniformity and reproduction, and if it is greater than about 25 microns, it is difficult to form a uniform paper having a low to medium basis weight. The short fibers are usually made by cutting filaments of continuous yarn or tow into pieces of specific length using conventional fiber cutting equipment.

The term "pulp", as used herein, means particles of aramid material having a rod and microscopic fibrous particles that extend generally from this, where the rod is generally in the form of a column and approximately 10 to 50 micrometers in diameter and the microscopic fibrous particles are fine, the hair-like members generally attached to the rod measuring only a fraction of a micrometer or a few micrometers in diameter and approximately 10 to 100 micrometers in length. A possible illustrative process for making aramid pulp is generally disclosed in U.S. Patent No. 5,084,136. The papers of this invention comprise from 5 to 65 parts by weight of aramid fiber, and in some embodiments from 30 to 50 parts by weight are preferred. It is believed that less than 5 parts by weight causes paper that is too bright and does not have sufficient tear properties, while papers having more than 65 parts by weight of aramid fibers cause a corresponding reduction in the amount of microscopic fibrous particles available. in the composition to help bind the composition together, which results in an unacceptable reduction in the tensile strength of the paper. In some embodiments of this invention, preferred types of the fiber useful in this invention are short fibers of poly (metaphenylene isophthalamide), pulp of poly (para-phenylene terephthalamide), and short fiber of poly (para-phenylene terephthalamide), with short fiber of poly (meta-phenylene isophthalamide), which is the most preferred fiber. The papers of this invention also comprise microscopic fibrous particles of aramid. The term "microscopic fibrous particles" as used herein, means a polymer product finely divided into small particles, essentially two-dimensional film having a length and a width in the order of 100 to 1000 microns and a thickness of the order of only 0.1 to 1 micrometer. The microscopic fibrous particles are made by passing a stream of the polymer solution into a coagulation bath of the liquid that is miscible with the solvent in the solution. The polymer solution stream is subjected to strenuous cutting forces and turbulence while the polymer coagulates. Microscopic fibrous aramid particles can be prepared using a device for forming microscopic fibrous particles where a polymer solution is precipitated and tuned in a single step as described in US Pat. No. 3,756,908 or 3,018,091. The papers of this invention comprise from 30 to 90 parts by weight of microscopic fibrous particles of aramid. It is believed that these papers having less than 30 parts by weight of microscopic fibrous particles do not have adequate tensile strength for most preferred applications, while papers having more than 90 parts by weight are not only typically more bright and do not have enough tear properties for most processing steps, but also this paper with high content of microscopic fibrous particles has a very limited resin impregnation even with low density. In some embodiments, the papers of this invention preferably have a microscopic fibrous aramid content of about 65 to 60 parts by weight. In some embodiments of this invention, the preferred microscopic fibrous aramid particles of this invention are made of meta-aramid polymer, with poly (meta-phenylene isophthalamide) being the most preferred of the meta-aramid. The aramid fiber and the microscopic fibrous particles useful in the paper of this invention may be natural color of the spinning filament or may be colored with dyes or pigments. The fiber can also be treated with materials that alter its surface characteristics as long as this treatment does not adversely affect the ability of the binders to contact and hold the fibrous surfaces. The papers of this invention also include a conductive filler. "Conductive filler" means any fibrous or particulate form (such as a powder or a flake) that has a conductivity over a wide range, such as a typical conductivity for conductors of more than 102 siemens / meter, to a typical conductivity for semiconductors from approximately 10 ~ 8 to 102 siemens / meter). The structure of the conductive filler can be chosen based on the requirements of the particular application and the conductive filler can be relatively homogeneous, where substantially all the volume of the material can conduct electricity (such as metallic fibers, carbon fibers, carbon black, etc.). .) or the materials may be heterogeneous where the conductive and dielectric part co-exist in the volume of the material (such as fibers or particles coated with metal, or fibers or particles filled with conductive ingredients). The papers of this invention comprise from 1 to 20 parts by weight of the conductive filler. It is believed that less than 1 part by weight originates a paper that does not provide an adequate amount of conduction for many applications, while having more than 20 parts by weight usually results in a noticeable reduction in the mechanical properties of the paper. In some preferred embodiments the conductive filler is carbon fiber, and in other preferred embodiments the conductive filler is carbon black. The most preferred conductive filler in many versions of the inventive paper is carbon fiber. The paper of this invention has a bulk density of no greater than 0.43 g / cm 3 and a tensile index no greater than 60 Nm / g. These papers can be used in any protection or interference discharge application and can easily be tape wrapped and impregnated with a resin. The bulk density describes the weight to volume ratio of the paper and is determined according to the ASTM D202 method. The Tension Index describes the ratio of the tensile strength to the basis weight (grammage) and is determined according to the ASTM method D828. In some embodiments of this invention, the papers of this invention have a final basis weight of about 30 to 60 g / m2 and have a final thickness of about 0.08 to 0.16 mm. The papers of this invention are generally impregnated with resins either before or after being installed in / on the electrical or conductive device. These resins include epoxy resins, polyesterimide resins, and other resin systems. It has been found to be critical that the papers of this invention have a bulk density of no greater than 0.43 g / cm3paar to form and allow rapid impregnation with typical resins. A higher density provides a structure that is too consolidated to form or to allow rapid impregnation. In addition, it is thought that the apparent density of the paper can be as low as 0.15 g / cm 3 or less, depending on the application, the resin used, and the amount of resin used. To prevent electrical discharges in electrical machinery, tapes made from this paper of the invention are generally applied to the conductive coils using automated tape winding machinery, and it has been found that a tension index of not greater than 60 Nm / g It is necessary to avoid breaking or excessive tearing of the papers in these machines. Additional ingredients, such as fillers for adjusting the strength of paper and other properties, or pigments or antioxidant, etc., in powder, flake, or fibrous form can be added to the paper composition of this invention, these do not they affect increasing the apparent density or reduce the tension index to unacceptable levels. This invention also relates to a process for making aramid paper, comprising the steps of: a) forming an aqueous dispersion of 5 to 65 parts by weight of aramid fiber, 30-90 parts by weight of microscopic fibrous particles of aramid , and 1-20 parts by weight of the conductive filler, based on the total weight of the aramid fiber, microscopic fibrous particles, and filler. b) Mix the dispersion to form a slurry, c) draining the water from the slurry to produce a wet paper composition, d) drying the wet paper composition, and e) heat-treating the paper at or above the glass transition temperature of the polymer in the microscopic fibrous particles of Aramid without paper consolidation. The first step of this invention involves forming a dispersion of aramid fiber, microscopic fibrous particles of aramid and conductive filler in an aqueous liquid such as water. The dispersion can be done either by dispersing the fibers and then adding the microscopic fibrous particles and other materials or by dispersing the microscopic fibrous particles and then adding the fibers and other materials. The dispersion can also be done by combining a first dispersion of the fibers with a second dispersant of the microscopic fibrous particles and other materials. Any number of possibilities of combining the fibers, microscopic fibrous particles and other materials is possible, however, in a preferred embodiment the concentration of the fibers in the final dispersion is from about 0.01 to 1.0 weight percent based on the total weight of the total weight of the dispersion. In other preferred embodiments, the concentration of the microscopic fibrous particles in the dispersion is up to about 95 weight percent based on the total weight of the solids. The aqueous liquid of the dispersion is generally water, but may include other different materials such as pH adjusting materials, formation aids, surfactants, defoamers, and the like. The second step in the papermaking process of this invention mixes the dispersion to form a slurry. The dispersion can be mixed in one step or container completely separately or the dispersion can be mixed essentially simultaneously while forming, and the mixture can be made in the same vessel that forms the dispersion. The mixing can be carried out by any known means, such as agitation of the dispersion, i.e., a stirring device, or by refining the dispersion in a refiner, or in some embodiments mixing can be performed by pumping the dispersion with a speed to provide the proper turbulence to mix the materials. The third step in the process for making the paper of this invention involves draining the aqueous liquid from the second slurry to produce a paper composition. In some embodiments, the aqueous liquid is drained from the dispersion by conducting the dispersion in a sieve or other perforated support, retaining the dispersed solids and then passing the liquid to produce a wet paper composition. For example, the papers of this invention can be formed into equipment that can be scaled from laboratory screens to commercial size paper forming machinery, such as a Fourdrinier or flat table forming machine. The next step in the process for making the paper of this invention involves drying the wet paper composition. In different embodiments of the process of this invention, the wet paper composition, once formed on the support or screen, is further dehydrated by vacuum or other pressure forces and dries more by evaporating the remaining liquid using a dryer, oven or similar device known in the art for drying fabrics and papers. The final step in the process for making the paper of this invention involves the thermal treatment of the paper at or above the glass transition temperature of the polymer in the microscopic fibrous particles without consolidating the paper. The vitreous transition temperature of the poly (m-phenylene isophthalamide) is about 275 ° C. The heat treatment can be carried out in line with the formation or as a separate processing step. Surprisingly, the inventors have found that a tough paper without significant changes in free paper volume or surface resistivity can be made by heat treating the paper formed at a temperature of about or above the glass transition temperature of the aramid polymer of the microscopic fibrous particles but without applying substantial pressure to the sheet in the hot state to consolidate or compress the paper. Thus, this process does not involve any of the preliminary calendering steps of compression or compression to consolidate the structure of the sheet as it is in typical processes of the prior art. If desired, the paper can be stretched again while it is heat treated to help reduce wrinkling. The heat treatment may be carried out by any known method of heating including, but not limited to heating by contact with the paper touching the hot surface of metal rollers or other hot surfaces, by means of conventional heating such as infrared or infrared heating. hot air in an oven. The paper of this invention is useful as a conductive material with a designed level of electrical properties for the interference of electrostatic discharge and / or protection against electromagnetic interference. For example, it can be used as a conductive tape for electrostatic discharge in the slots of the stators of high voltage rotating machines.

Test Methods Thickness and Base Weight (grammage) was determined for papers of this invention according to the method ASTM D 374 and ASTM D 646 correspondingly. In thickness measurements, he used the compression method E on the specimen of approximately 172 kPa.

Density (apparent density) was determined on the papers according to the ASTM D202 test method.

Voltage index was determined based on the stress test in an Instron type test machine using test specimens 2.54 cm wide and a length of 12.7 cm gauge according to the method ASTM D 828.

Surface resistivity was measured according to the ASTM D 257 method on strips with a width of approximately 2.54 cm from the paper.

Examples Physical properties of all paper samples in the examples are shown in the Table.

Example 1 An aqueous dispersion of microscopic fibrous particles of poly (meta-phenylene isophthalamide) (MPD-I) was made with a consistency of 0.5% (0.5 weight percent solids in water). Carbon fiber was added to this dispersion. After about 10 minutes of continuous stirring, additional water and short meta-aramid fiber were added with additional stirring of about 10 minutes to thoroughly mix the materials and to produce a slurry having a final consistency of 0.35%. The final slurry was comprised of the following weight solids: 39% short fiber MPD-I, 50% microscopic fibrous particles MPD-I and 11% carbon fiber. The microscopic fibrous particles of MPD-I were made using the general method as disclosed and described in U.S. Pat. No. 3,756,908. The short fiber of MPD-I had a linear density of 2.0 denier (0.22 tex) of 0.64 cm, and an initial modulus of approximately 800 cN / tex (sold by DuPont under the trademark NOMEX®.) Carbon fiber was fiber FORTAFIL type 150 (length 0.32 cm), available from FORTAFIL Inc. The slurry was pumped into a supply tray and fed from it to a Fourdrinier machine to make paper having a basis weight of approximately 30.9 g / m2. paper was then thermally treated by surface contact on the hot metal rolls having a surface temperature of about 320 ° C and a contact residence time of about 7 seconds, a 2 cm wide ribbon was made with this paper which was rolled up successfully without breaking or tearing on a coil using an automated rolling process.

Example 2 A slurry was prepared as in Example 1, however, the final slurry was comprised of the following solids by weight: 40% short fiber MPD-I, 50% microscopic fibrous particles MPD-I and 10% of carbon fiber. A paper with a basis weight of 50.2 g / m2 was formed on a Fourdrinier machine and thermally treated additionally as in Example 1. A 2 cm wide ribbon of this paper was successfully wound without breaking or tearing on a coil using an automated rolling process.

Example 3 A slurry was prepared as in Example 1, however the final slurry was comprised of the following solids by weight: 44% short fiber MPD-I, 50% microscopic fibrous particles MPD-I and 6% carbon fiber. A paper with a basis weight of 53.9 g / m2 was formed on a Fourdrinier machine and was thermally treated additionally as in Example 1. A 2 cm wide ribbon of this paper was successfully wound without breaking or tearing on a coil using an automated rolling process.

Example 4 A slurry was prepared as in Example 1, however the final slurry was comprised of the following solids by weight: 60% short fiber MPD-I, 40% microscopic fibrous particles MPD-I and 10% carbon fiber. A paper with a basis weight of 45.8 g / m2 was formed on a Deltaformer inclined cable machine and was thermally treated additionally as in Example 1. A 2 cm wide ribbon of this paper was successfully wound without breaking or tearing on a coil using an automated coiling process.

Example 5 172 g of an aqueous slurry, never dried, of fibrous meta-aramid particles (0.58% consistency and refining 330 ml of Shopper-riegler), 0.34 g of carbon black and 0.66 g of short metharamide fiber it was placed together in a laboratory mixer (British octopus evaluation device) with approximately 1600 g of water and stirred for 1 minute. The final slurry was comprised of the following weight solids: 33% short fiber MPD-I, 50% microscopic fibrous particles MPD-I and 17% carbon black. The short fiber MPD-I and the microscopic fibrous particles MPD-I were the same as described in Example 1. The carbon black was Ketjenblack®EC300J produced by Akzo Nobel Co. The dispersion was emptied, with 8 liters of water, into a test sheet mold and a wet sheet was formed. The sheet was placed between the two pieces of blotting paper, passed through a couche roller with a barbed cylinder and dried in a laboratory sheet dryer at 190 ° C. After drying the sheet was heat treated in a stretched position again (fixed by metal staples to a metal plate) in an oven at 300 ° C for 20 minutes.

Comparative Example A It was prepared as in Example 5, but with additional heat treatment after drying. As a result, the Index of paper tension decreased significantly more below than necessary for the automated rolling operation.

Comparative Example B It was prepared as in Example 5, but instead of additional heat treatment after drying, the sheet was passed through a roller contact line of a metal-metal calender with a roller diameter of about 20 mm. cm with a temperature of about 300 ° C and a linear pressure of about 3000 N / cm.

Comparative Examples C-F Papers were formed as described in Examples 1-4 correspondingly, but no additional heat treatment was performed. During the automated rolling of 2 cm wide tapes of these papers, the breakage was presented.

Comparative Example G The paper of Example 1 was passed through a roller contact line of a metal-metal calender with a roll diameter of about 20 cm at a temperature of about 300 ° C and a linear pressure of about 1200 N / cm.

Comparative Example H The paper of Example 2 was calendered on a soft nip calender between rolls at an ambient temperature and linear pressure of about 870 N / cm, and was heat treated to the same conditions as described in Example 1. As can be seen from Table 1, the Stress Index of the inventive paper (Examples 1-5) ranges from 61 to 87 N / cm, which is close to the Stress Index for the calendered paper of the same composition (examples B, G &H) which has range from 68-85; however, the apparent density values for the inventive papers (Example 1-5) have a range between 0.28 to 0.41 g / cm 3 are almost the same as for the formed precursor paper represented by Examples A & C-F, with interval between 0.27 to 0.40 g / cm3. The surface resistivity of the inventive papers is also very close to the surface resistivity of the formed precursors (compared to Examples 1 and C, 2 and D, 3 and E, 4 and F, 5 and A). The great difference in resistivity for the papers formed and the thermally treated papers formed is the pair of Examples 3 and E (the change in approximately 2.4 times), but it is still much smaller than after the calendering (described below). Examples G and H illustrate that the surface resistivity of the carbon fiber calendered papers is much greater than the resistivity of the formed precursors represented by Examples C and D or formed and the heat treated paper represented by Examples 1 and 2. Examples A and B illustrate that the surface resistivity of calendered paper with carbon black (Example B) is 10 times less than the resistivity of the corresponding forming precursor (Example A). This reaction, which is different from that of papers made with carbon fibers, is believed to be due to the brightness of the carbon fiber and there is significant crushing and reduction in the length of these fibers when they are compressed in the contact line between rollers, causing a corresponding increase in surface resistivity. This effect may be less pronounced for stronger papers, but for lightweight paper, which is practically important (60 g / m2 and smaller), this is a very negative factor. Also, more uniform paper formation can reduce the scale of the effect; however, the economic issues of papermaking always limit this opportunity. In the case of these conductive powder fillers as a carbon black it is believed that there is a significant reduction in the resistivity of the paper after calendering due to the higher volume concentration of the conductive elements of the structure (ie the particles) without No change in your individual size. The main problem with the calendering of papers with both types of conductive fillers (carbon fiber and carbon black), as shown in the examples, is the dramatic change in surface resistivity after calendering.

Table 1 Properties of the papers 15 It is noted that in relation to this date the best method known to the applicant to carry out the aforementioned invention, is that it is clear from the present description of the invention.

Claims (6)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. Aramid paper, characterized in that it comprises from 5 to 65 parts by weight of aramid fiber, 30-90 parts by weight of microscopic fibrous particles of aramid, and 1-20 parts by weight of the conductive filler, based on the total weight of the aramid fiber, microscopic fibrous particles, and filler, the paper having a bulk density not greater than 0.43 g / cm3 and an Index of voltage that is not higher than 60 Nm / g.
2. 2. Aramid paper in accordance with the claim 1, characterized in that the filling is carbon fiber.
3. 3. Aramid paper according to claim 1, characterized in that the aramid fiber is poly (meta-phenylene isophthalamide) fiber.
4. 4. Aramid paper in accordance with the claim 2, characterized in that the aramid fiber is poly (meta-phenylene isophthalamide) fiber.
5. Process for making aramid paper, characterized in that it comprises the steps of: a) forming an aqueous dispersion of 5 to 65 parts by weight of aramid fiber, 30-90 parts by weight of microscopic fibrous particles of aramid, and 1- 20 parts by weight of the conductive filler, based on the total weight of the aramid fiber, microscopic fibrous particles, and filler. b) mixing the dispersion to form a slurry, c) draining the water from the slurry to produce a wet paper composition, d) drying the wet paper composition, and e) heat treating the paper at or above the transition temperature. vitrea of the polymer in the microscopic fibrous particles of aramid without consolidation of the paper.
6. 6. Process according to claim 5, characterized in that the water is drained from the second slurry by means of a screen or conveyor belt.