Low shedding aramid paper containing mica.

Abstract

1. An aramid paper suitable for use as an electrical insulator comprising: a first outer layer and a second outer layer, each of which comprises 70-30 weight percent aramid floc and 30-70 weight percent aramid fibrids, each of which is free of mica and has a first side and a second side; and an inner layer comprising 50-70 weight percent aramid material and 30-50 weight percent mica, and having a first side and a second side, wherein the first side of the first outer layer is made of the aramid paper. an aramid paper having a first outer surface, the second surface of the first outer layer being coextensive with the first surface of the inner layer, in the first outer layer and in the inner layer, and bonded face-to-face to the first surface of the inner layer only by fibrids, and the first surface of the second outer layer being coextensive with the second surface of the inner layer, in the second outer layer and in the inner layer, and bonded face-to-face to the second surface of the inner layer only by fibrids, the second surface of the second outer layer being a second outer surface of the aramid paper, the aramid paper having a total of 25 to 40 weight percent mica.

Description

[Technical field]

[0001] The present invention relates to aramid papers suitable for use, particularly but not exclusively, as electrical insulation in articles such as motors, generators, transformers, and inverters. [Background technology]

[0002] U.S. Patent No. 9,437,348 to Turpin et al. discloses a nonwoven paper layer directly fused on one or both sides with a nonwoven fabric layer, where one or both of the nonwoven paper and nonwoven fabric are electrically insulating, and the nonwoven fabric is a sheet material composed primarily of long fibers, i.e., fibers one inch or longer in length.

[0003] Various patent publications disclose the use of mica in aramid paper. Such publications include U.S. Patent No. 6,991,845, U.S. Patent No. 7,399,379, and U.S. Patent No. 9,073,290 to Levit et al., U.S. Patent No. 6,312,561 to Forsten et al., U.S. Patent No. 10,336,039 and U.S. Patent No. 10,186,353 to Kang et al., and U.S. Patent Publication No. 20130196161, U.S. Patent No. 9,972,419 to Kang, and U.S. Patent No. 9,844,928 to Duart et al.

[0004] The crystalline structure of mica gives the material excellent electrical properties as an insulator, high dielectric breakdown, thermal stability up to 500 °C (932 °F), resistance to corona discharges, and the ability to maintain its electrical properties even in small particles.

[0005] However, the presence of mica can reduce the mechanical strength and cohesion of the paper. However, combining mica paper with a nonwoven, film, or glass scrim support to improve mechanical performance is undesirable as it requires additional steps and various adhesives, etc., and can increase the thickness of the paper, reduce electrical performance, or introduce other issues such as chemical compatibility with motor coolants. Additionally, the industry hopes that the improved paper will be an interim replacement for the very thin electrical insulation papers currently in use.

[0006] In addition, the manufacture of motors and related equipment is now commonly automated using machines that quickly and automatically insert electrical insulation into slots and other areas of the motor where wear between the motor parts and the electrical insulation can cause the electrical insulation to buckle or fall apart. In such processes, it has been found that this wear can cause papers containing mica to shed mica particles from the paper, creating dusting problems. Summary of the Invention [Problem to be solved by the invention]

[0007] Therefore, there is a need for an aramid paper containing mica that has sufficient strength and also has good resistance to particle shedding. [Means for solving the problem]

[0008] The present invention relates to an aramid paper suitable for use as an electrical insulator, the aramid paper comprising: a) a first outer layer comprising 70 to 30 weight percent aramid floc and 30 to 70 weight percent aramid fibrids, the first outer layer being free of mica and having a first surface and a second surface; b) an inner layer comprising 50-70 weight percent aramid material and 30-50 weight percent mica, the inner layer having a first side and a second side; c) a second outer layer comprising 70 to 30 weight percent aramid floc and 30 to 70 weight percent aramid fibrids, the second outer layer being free of mica and having a first surface and a second surface; Including, The first surface of the first outer layer is the first outer surface of the aramid paper, the second surface of the first outer layer is coextensive with the first surface of the inner layer in the first outer layer and in the inner layer and is bonded face-to-face to the first surface of the inner layer only by fibrids, the first surface of the second outer layer is coextensive with the second surface of the inner layer in the second outer layer and in the inner layer and is bonded face-to-face to the second surface of the inner layer only by fibrids, and the second surface of the second outer layer is the second outer surface of the aramid paper, the aramid paper having a total of 25 to 40 weight percent mica. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The present invention relates to an aramid paper suitable for use as an electrical insulator comprising a first outer layer and a second outer layer, each of which comprises 70-30 weight percent aramid floc and 30-70 weight percent aramid fibrids, each of which is further free of mica (does not contain mica) and having a first side and a second side, and an inner layer sandwiched between the first outer layer and the second outer layer, the inner layer comprising 50-70 weight percent aramid material and 30-50 weight percent mica, and also having a first side and a second side. The first side of the first outer layer is the first outer side of the aramid paper, and the second side of the first outer layer is coextensive with the first side of the inner layer in the first outer layer and inner layer and is bonded opposite to the first side of the inner layer only by the fibrids. Similarly, the first surface of the second outer layer is coextensive with the second surface of the inner layer in the second outer and inner layers and is bonded face to face with the second surface of the inner layer only by fibrids, and the second surface of the second outer layer is the second outer surface of the aramid paper. The aramid paper formed from a three-layer structure (mica-free outer layer / mica-containing inner layer / mica-free outer layer) further comprises a total of 25 to 40 weight percent mica.

[0010] The mica-free outer layer, the mica-containing inner layer, and the mica-free outer layer are bonded together only by the aramid fibrids present on the faces of each layer. The fibrids are uniformly distributed in each layer, with some of the fibrids present on the surface of each layer, i.e., the fibrids are present on both sides of each layer. Thus, the fibrids are available on the surface of each layer, forming an intimate contact between the layers and bonding or fusing the faces of the two layers together when the layers are compressed together under high temperature and pressure. This compression or consolidation can be accomplished in a static heating press or a set of calendar rolls forming a nip, in a process that can compress or consolidate the sheets at a temperature above the glass transition temperature of the fibrids. The use of fibrids in the various layers to bond the layers together eliminates the need for additional adhesives or other binders that not only require a separate step to apply, but may also be chemically incompatible with motor cooling and other fluids to which the electrical insulator may be exposed during use.

[0011] As used herein, aramid paper refers to a planar sheet made from plies or layers of aramid material produced by a papermaking process. Representative equipment and machines that can be used to produce the plies or layers include, for example, but are not limited to, continuous processing equipment such as field linear paper machines or inclined wire machines, or batch processing equipment such as processing equipment that produces paper manually in a handsheet mold that includes a forming screen. Specifically, aramid paper is a planar sheet that includes an aramid layer that includes mica having first and second opposing planar surfaces, each of which has a mica-free aramid layer attached thereto. Thus, the aramid paper can be composed of an outer mica-free aramid layer, an inner mica-containing aramid layer, and an outer mica-free aramid layer laminated together in that order.

[0012] Aramid paper, which is made by laminating an aramid layer without mica, an aramid layer with mica, and an aramid layer without mica in this order, contains a total of 25 to 40 weight percent mica based on the total weight of mica in the total weight of the aramid paper. If the amount of mica in the aramid paper is less than 25 weight percent, the measured partial discharge inception voltage of the aramid paper is considered to be too low for many applications. The partial discharge inception voltage (PDIV) of the paper is the level of applied voltage at which localized dielectric breakdown of the paper begins. Thus, the partial discharge inception voltage (PDIV) and partial discharge inception voltage per unit thickness (PDIV / mil) are important properties because higher values ​​are desirable and lower values ​​mean that rapid degradation of the insulating material during use may occur, which may lead to reduced insulation life and / or insulation failure. Similarly, if the amount of mica in the aramid paper exceeds 40 weight percent, the mechanical properties of the paper may decrease to the point where the aramid material is not suitable for many desired uses.

[0013] In some other embodiments, the aramid paper formed by laminating an aramid layer without mica, an aramid layer with mica, and an aramid layer without mica in this order has a total of 30 to 40 weight percent of mica based on the total weight of the mica in the total weight of the aramid paper. In some further embodiments, the aramid paper formed by laminating an aramid layer without mica, an aramid layer with mica, and an aramid layer without mica in this order has a total of 35 to 40 weight percent of mica based on the total weight of the mica in the total weight of the aramid paper.

[0014] The aramid material may include aramid floc. As used herein, the term "floc" refers to fibers cut into short lengths that are typically used in the manufacture of paper. Typically, the floc has a length of about 3 to about 20 millimeters. A preferred length is about 3 to about 7 millimeters. Floc is typically produced by cutting continuous fibers to the required length using methods known in the art.

[0015] As used herein, the term "aramid" refers to an aromatic polyamide in which at least 85% of the amide (-CONH-) linkages are directly attached to two aromatic rings. Optionally, additives may be used with the aramid and may be dispersed throughout the polymer structure. It has been found that up to about 10 weight percent of other polymeric materials can be blended with the aramid. It has also been found that copolymers having up to about 10 percent of other diamines replacing the diamines of the aramid, or up to about 10 percent of other diacid chlorides replacing the diacid chlorides of the aramid, can be used.

[0016] A preferred aramid is a meta-aramid. An aramid polymer is considered to be a meta-aramid when two rings or radicals are oriented meta to each other along the molecular chain. A preferred meta-aramid is poly(meta-phenylene isophthalamide) (MPD-I). U.S. Pat. Nos. 3,063,966, 3,227,793, 3,287,324, 3,414,645, and 5,667,743 describe useful methods for producing aramid fibers that can be used to produce aramid floc.

[0017] Aramid materials can include aramid fibrids. As used herein, the term "fibrids" refers to very small non-granular, fibrous or film-like particles with at least one of their three dimensions being an order of magnitude smaller than the largest dimension. These particles are produced by precipitating a solution of a polymeric material with a non-solvent under high shear. Aramid fibrids are non-granular film-like particles of aromatic polyamides with a melting or decomposition point above 320°C. Preferred aramid fibrids are meta-aramid fibrids, and particularly preferred are fibrids made from meta-aramid poly(meta-phenylene isophthalamide) (MPD-I).

[0018] Fibrids generally have a maximum dimension length in the range of about 0.1 mm to about 1 mm with a length to width aspect ratio of about 5:1 to about 10:1. Thickness dimensions are on the order of a fraction of a micron, e.g., about 0.1 micron to about 1.0 micron. Although not required, it is preferred to incorporate the aramid fibrids into the layer while they are in a wet state.

[0019] The aramid paper has a first outer layer or ply comprising 70-30 weight percent aramid flock and 30-70 weight percent aramid fibrids, the first outer layer being free of mica and having a first side and a second side. Similarly, the aramid paper has a second outer layer or ply comprising 70-30 weight percent aramid flock and 30-70 weight percent aramid fibrids, the second outer layer being free of mica and having a first side and a second side. It is believed that these weight ranges provide the best mechanical properties for the final aramid paper. Mechanical strength is required so that the aramid paper does not buckle or fall apart during automated insertion into equipment such as slot liners in electric motors. In addition, it is believed that these weight ranges are necessary to provide the paper with the desired abrasion resistance during use. The first and second outer layers act synergistically within the aramid paper to not only eliminate mica shedding by covering the mica containing layer or ply, but also provide mechanical stability to avoid damage to the paper during use that could result from the release of mica particles.

[0020] In some embodiments, the first and second outer layers of the aramid paper each contain 60-40 weight percent aramid floc and 40-60 weight percent aramid fibrids. In some preferred embodiments, the first and second outer layers of the aramid paper each contain 50-40 weight percent aramid floc and 50-60 weight percent aramid fibrids. In some embodiments, the first and second outer layers of the aramid paper have the same composition. In yet other embodiments, the first and second outer layers of the aramid paper consist only of a mixture of aramid floc and aramid fibrids.

[0021] The term "layer" refers to a thin planar material preferably produced on a paper machine in the form of a formed web. In most papermaking, an aqueous dispersion containing the desired paper composition is fed to a screen where the solid materials in the dispersion form what is sometimes known as a wet-laid web, or waterleaf, and the water is removed by gravity, vacuum, and / or press. The wet-laid web, when dried, becomes a "formed web" as used herein. In the art, the formed web is sometimes also referred to as a "ply" or a dry waterleaf. Thus, as used herein, the term "layer" preferably refers to a formed web, ply, or dry waterleaf, and these terms are used interchangeably. Also, as used herein, the term "face" refers to either of the two major faces of the layer (i.e., one side or the other side of the layer).

[0022] In one embodiment, each of the first and second outer layers has a thickness of 0.001 to 0.003 inches (1 to 3 mils), and in a preferred embodiment, each of the first and second outer layers has a thickness of 0.0015 to 0.002 inches (1.5 to 2 mils).

[0023] The aramid paper also has an inner layer or ply that includes 50-70 weight percent aramid material and 30-50 weight percent mica, the inner layer having a first side and a second side. In some embodiments, the aramid material of the inner layer is a combination of less than 50 weight percent aramid floc and more than 50 weight percent aramid fibrids. In some specific embodiments, the aramid material of the inner layer is a combination of 15-45 weight percent floc and 55-85 weight percent fibrids.

[0024] The mica is usually used in the form of flakes and can be of various types, such as muscovite or phlogopite, or blends thereof, however, muscovite types of mica are preferred.

[0025] It is believed that if the mica-containing inner layer has more than 50 weight percent mica, the properties of the final laminate structure are adversely affected. First, as the amount of mica in a layer increases on a constant weight basis, the tensile strength of the layer decreases. Second, it is believed that 50 weight percent or less of mica in the mica-containing inner layer prevents bridging of the mica within the layer, which promotes dispersion of the mica throughout the layer. Furthermore, it is believed that if the mica-containing inner layer has less than 30 weight percent mica, there is not enough mica in the layer to provide the desired electrical properties.

[0026] In one embodiment, the inner layer comprising mica has a thickness of 0.002 to 0.010 inches (2 to 10 mils), and in a preferred embodiment, the inner layer comprising mica has a thickness of 0.004 to 0.006 inches (4 to 6 mils).

[0027] The first surface of the first outer layer is the first outer surface of the aramid paper, and the second surface of the first outer layer is coextensive with the first surface of the inner layer in the first outer and inner layers and is bonded face-to-face with the first surface of the inner layer only by fibrids. Similarly, the first surface of the second outer layer is coextensive with the second surface of the inner layer in the second outer and inner layers and is bonded face-to-face with the second surface of the inner layer only by fibrids, and the second surface of the second outer layer is the second outer surface of the aramid paper.

[0028] The phrase "coextensive with and face-to-face bonded to" preferably means that each outer layer and inner layer have the same planar boundary, the edges of the outer and inner layers are the same, and neither outer layer extends beyond the edges of the inner layer, or vice versa.

[0029] The phrase "oppositely bonded" preferably means that each opposing surface of the inner layer is bonded to one surface of each outer layer. In some embodiments, the surfaces are uniformly bonded, meaning that the attachment of each surface of the inner layer to the surface of each outer layer is substantially uniform across the entire planar surface. By substantially uniform, it is meant that the layers are bonded without visual gaps in the attachment or visually detectable discontinuous areas of non-attachment. In some embodiments, the surfaces are continuously bonded, meaning that the entire surface of each surface of the inner layer is attached to the entire surface of one of the surfaces of the outer layers.

[0030] The aramid paper can be produced in a batch process involving several steps using manufacturing techniques and conditions such as those described in GB 1,129,097, US 2010 / 0122769, and US 4,481,060, or more preferably, can be produced continuously or semi-continuously by combining two mica-free outer layers (or plies) with an inner layer (or ply) that contains mica.

[0031] In some embodiments, the aramid paper, which is made by laminating an aramid layer without mica, an aramid layer with mica, and an aramid layer without mica in that order, has a total thickness of 0.004 to 0.016 inches (4 to 16 mils). In some embodiments, aramid paper that is particularly suitable for use at many high voltages (600 to 800 volts) has a total thickness of 0.007 to 0.012 inches (7 to 12 mils), preferably a total thickness of 0.008 to 0.010 inches (8 to 10 mils). In some other embodiments, for example, for applications where the voltage requirements may be lower, the aramid paper has a total thickness of 0.004 to 0.007 inches (4 to 7 mils).

[0032] In addition, the aramid paper formed by laminating a first mica-free aramid outer layer, a mica-containing aramid inner layer, and a second mica-free aramid outer layer in this order has a total thickness of 0.004 to 0.016 inches (4 to 16 mils), and the mica-containing inner layer has a thickness of 0.002 to 0.010 inches (2 to 10 mils), so long as the mica-containing inner layer has a thickness of 0.002 to 0.010 inches (2 to 10 mils), the mica-containing inner layer can be formed from two or more mica-containing aramid sub-plies or sub-layers, preferably two or more identical mica-containing aramid sub-plies or sub-layers. Similarly, an aramid paper having a first mica-free aramid outer layer, a mica-containing aramid inner layer, and a second mica-free aramid outer layer laminated together in that order has a total thickness of 0.004 to 0.016 inches (4 to 16 mils), and each of the first and second mica-free aramid outer layers has a thickness of 0.001 to 0.003 inches (1 to 3 mils), so long as each of the mica-free aramid outer layers has a thickness of 0.001 to 0.003 inches (1 to 3 mils). Each of the mica-free aramid outer layers can be formed from two or more mica-free aramid sub-plies or sub-layers, preferably two or more identical mica-free aramid sub-plies or sub-layers. Preferably, the aramid paper is made by combining all the individual layers, and / or any sub-plies or sub-layers (if present), and then consolidating all of the layers together at once.

[0033] Aramid paper having a mica-free aramid layer, a mica-containing aramid layer, and a mica-free aramid layer laminated in that order provides a structure that effectively shields the relatively weaker mica-containing inner layer with a durable mica-free outer layer to prevent damage to the inner layer and the associated shedding of the mica when the aramid paper is installed in various devices that the aramid paper is used in. Shedding of the aramid paper can be simulated using a Taber abrasive machine that continuously processes the surface of the paper over many cycles, simulating the wear of the surface during use by automated equipment. The mica-free outer layer protects the mica-containing inner layer so well that in some embodiments, when the aramid paper is subjected to a Taber abrasion test after 125 cycles using a weight of 1000 g on each arm (using a Taber 5150 Abraser (Abrader) and H018 wheels from Taber Industries, Tonawanda, NY), the weight loss in grams of the aramid paper is less than or equal to the weight loss of a mica-free aramid paper of the same thickness similarly tested. Thus, aramid paper containing significant amounts of mica (25-40 weight percent) unexpectedly has shedding performance similar to that of a mica-free sheet.

[0034] Test Method Thickness was measured using ASTM-D374(2010).

[0035] The partial discharge inception voltage (PDIV) was measured according to ASTM-D1868(2020).

[0036] The partial discharge inception voltage per thickness (PDIV / mil) was calculated by dividing the partial discharge inception voltage (PDIV) by the thickness (in mils) of a particular example.

[0037] The tensile strength was measured according to ASTM-D828(2010).

[0038] Taber abrasion was measured on a Taber 5150 Abraser according to its instructions (https: / / www.taberindustries.com / taber-rotary-abraser). The paper measurement setup used a 1000g weight on each of the H-18 abrasion wheels and arms, and 125 cycles were run on each sample. The weight of the sample (not including any loose surface particles) both before and after the test was used to calculate the weight loss of the sample. EXAMPLES

[0039] Example 1 An aramid paper consisting of two mica-free outer web layers with a single mica-containing inner web layer was prepared as follows.

[0040] To make the mica-free layer or ply, a mixture of 55 weight percent poly(m-phenylene isophthalamide) (MPD-I) fibrids and 45 weight percent crystallized MPD-I floc was made in water to form an aqueous dispersion suitable as a papermaking furnish. The MPD-I fibrids were made as generally described in U.S. Pat. No. 3,756,908, and the MPD-I floc had a linear density of 0.22 tex and a length of 0.64 cm. This dispersion was fed to the headbox of a field linear papermaking machine to form a wet web or water leaf. The wet web was then dried to form an unconsolidated mica-free aramid formed web or layer having a thickness of 2 mils (0.002 inches). The papermaking machine speed was then slowed to produce a similar formed web or layer having a thickness of 3 mils (0.003 inches).

[0041] To make the mica-containing layer or ply, a mixture of 48 weight percent muscovite type mica, 37 weight percent (MPD-I) fibrids, and 15 weight percent crystallized MPD-I floc was made in water to form an aqueous dispersion containing mica suitable as a papermaking feed. The MPD-I fibrids and MPD-I floc were the same as those used in the mica-free layer. The mica-containing dispersion was fed to the headbox of a field linear paper machine to form a wet-laid web. The wet-laid web was then dried to form an unconsolidated mica-containing aramid formed web or layer having a thickness of 3 mils (0.002 inches). The paper machine speed was then slowed to produce similar mica-containing formed webs or layers having thicknesses of 4 mils (0.004 inches), 6 mils (0.006 inches), 8 mils (0.008 inches), and 9 mils (0.009 inches).

[0042] To illustrate the potential for shedding, a sample of aramid paper was prepared consisting of two mica-free outer web layers with a single mica-containing inner web layer sandwiched between the two outer layers. Specifically, the aramid paper was prepared by laminating the various layers by hand, consolidating the three layers together in a heated static press operating at a surface temperature of 280° C., and pressing the layers together at a pressure of 500 psi.

[0043] The properties of the resulting three-ply aramid paper are summarized in Table 1, item 1. Three comparative samples A, B, and C are also shown in Table 1. Sample A was a two-ply paper with only two 3 mil mica-free web layers and no mica-containing web layers. Sample B was a three-ply paper with three 3 mil mica-free web layers and no mica-containing web layers. Sample C was a paper made with only one thick mica-containing formed web or layer and no mica-free layers.

[0044] The samples were subjected to a Taber abrasion test to determine the amount of shedding that could occur. After 125 cycles, the samples were shaken to remove any loose surface particles. The samples were then weighed and compared to the sample weight before the test to determine the grams of material lost from the paper during the 125 cycles.

[0045] As shown, comparative aramid paper samples A and B without mica had very little shedding, 0.19 grams and 0.09 grams of particulate matter, respectively, which was clearly aramid fibrous material due to the absence of mica. The inventive aramid paper sample of item 1 had a similar level of particulate matter of 0.15, indicating essentially no mica shedding from the paper. Comparative aramid paper samples C and D show high levels of particulate shedding from the mica-containing paper without the outer protective layer.

[0046] [Table 1]

[0047] Example 2 Various samples of aramid paper were made using the mica-free and mica-containing layers of Example 1. As in Example 1, the aramid paper consisted of two mica-free outer layers with a single mica-containing inner layer sandwiched between the two outer layers. As in Example 1, the aramid paper samples were made by laminating the various layers by hand, consolidating the three layers together in a heated static press operating at a surface temperature of 280° C., and pressing the layers together at a pressure of 500 psi. The properties of the specific samples and the resulting three-ply paper are summarized in Table 1. Also shown in Table 2 for illustrative purposes is data for Sample C from Example 1, a paper made with only one thick mica-containing layer and no mica-free layer.

[0048] [Table 2]

[0049] Example 3 A sample of aramid paper having the composition of Table 2, item 3, consisting of two mica-free outer layers with a single mica-containing inner layer sandwiched between the two outer layers, was prepared as described above, except that the individual layers were fed from individual rolls, joined, and then consolidated together in a continuous process in the nip between heated calendar rolls operating at a surface temperature of 280° C. and pressing the layers together with a pressure of 1300 pounds per linear inch to consolidate the three layers to a thickness of about 0.0102 inches (10.2 mils). The PDIV of the aramid paper was 1327 volts, which corresponds to a PDIV / mil of 130 volts per mil, and the tensile strength was 98.6 lbs.

[0050] Reference Examples This example shows the adverse effect that the addition of mica can have on the mechanical properties of aramid sheets. To illustrate the effect of the addition of mica on sheet properties, single layer mica-free and mica-containing aramid handsheets were made. Aqueous dispersions were made as described in Example 1, and then individual handsheet samples were made using handsheet molds. Each aqueous dispersion was poured into a 21×21 cm handsheet mold with 8 liters of water to form five wet-laid papers R-0, R-20, R-30, R-40, and R-50, with item R-0 containing no mica. The handsheets were then individually placed between two pieces of blotter paper, hand-laid with a rolling pin, and dried in a handsheet dryer at 150° C. for 10 minutes. The final composition of the handsheets ranged from 0-50 weight percent mica, 37-65 weight percent MPD-I fibrids, and 13-35 weight percent aramid flock, as shown in Table 3. The initial unconsolidated caliper of all handsheets prior to pressing in the static press was nominally 0.005 inches (5 mils).

[0051] As shown in Table 3, while the addition of mica improves the partial discharge inception voltage of the handsheets, it has a significant detrimental effect on the mechanical properties of the sheets, most notably the tensile strength of the conventional mica-containing aramid handsheets.

[0052]

Table 3

Claims

1. An aramid paper suitable for use as an electrical insulator, a) A first outer layer comprising 70 to 30 weight percent aramid flock and 30 to 70 weight percent aramid fibrid, not containing mica, having a first surface and a second surface; b) An inner layer comprising 50 to 70 weight percent aramid material and 30 to 50 weight percent mica, having a first surface and a second surface; c) A second outer layer comprising 70 to 30 weight percent aramid flock and 30 to 70 weight percent aramid fibrid, not containing mica, having a first surface and a second surface; comprising, wherein the first surface of the first outer layer is the first outer surface of the aramid paper, the second surface of the first outer layer has the same extent as the first surface of the inner layer in the first outer layer and the inner layer, and is bonded to face the first surface of the inner layer by fibrid only; wherein the first surface of the second outer layer has the same extent as the second surface of the inner layer in the second outer layer and the inner layer, and is bonded to face the second surface of the inner layer by fibrid only, and the second surface of the second outer layer is the second outer surface of the aramid paper; the aramid paper having a total of 25 to 40 weight percent mica.

2. The aramid paper according to claim 1, wherein the aramid material of the inner layer is a combination of less than 50 weight percent aramid flock and more than 50 weight percent aramid fibrid.

3. The aramid paper according to claim 2, having 15 to 45 weight percent flock and 55 to 85 weight percent fibrid.

4. The aramid paper according to claim 1, wherein each of the first and second outer layers has a thickness of 0.001 to 0.003 inches.

5. The aramid paper according to claim 4, wherein each of the first and second outer layers has a thickness of 0.0015 to 0.002 inches.

6. The aramid paper according to claim 1, wherein the inner layer has a thickness of 0.002 to 0.010 inches.

7. The aramid paper according to claim 6, wherein each of the inner layers has a thickness of 0.004 to 0.006 inches.

8. The aramid paper according to claim 1, having a total thickness of 0.004 to 0.016 inches.

9. The aramid paper according to claim 8, having a total thickness of 0.007 to 0.012 inches.

10. The aramid paper according to claim 9, having a total thickness of 0.008 to 0.010 inches.

11. The aramid paper according to claim 8, having a total thickness of 0.004 to 0.007 inches.

12. The aramid paper according to any one of claims 1 to 11, having a weight loss in grams that is less than or equal to the weight loss of an aramid paper without mica of the same thickness tested similarly when the aramid paper is subjected to Taber abrasion measurement after 125 cycles using a 1000 g weight on each of the arms.

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