Low-shedding aromatic polyamide paper containing mica

Abstract

An aramid paper suitable for use as an electrical insulating material comprises: a first outer layer and a second outer layer, each of the first and second outer layers comprising 70 to 30 weight percent of aramid flocculent material and 30 to 70 weight percent of aramid fibrous material, and each of the first and second outer layers being mica-free and having a first side and a second side; an inner layer comprising 50 to 70 weight percent of aramid material and 30 to 50 weight percent of mica and having a first side and a second side; wherein The first outer layer has a first outer surface that is the first outer surface of the aramid paper, and the second outer surface of the first outer layer is co-extended with the first surface of the inner layer and is bonded together face-to-face only through the fibrous bodies in the first outer layer and the inner layer; and the second outer layer has a first surface that is co-extended with the second surface of the inner layer and is bonded together face-to-face only through the fibrous bodies in the second outer layer and the inner layer, and the second outer surface of the second outer layer is the second outer surface of the aramid paper; the aramid paper has a total of 25 to 40 percent mica by weight.

Description

Background Technology

[0001] This invention relates to an aramid paper, which is suitable, but not limited to, for use as an electrical insulation material in applications such as electric motors, generators, transformers, and inverters.

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

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

[0004] The crystal structure of mica provides a material with excellent electrical properties as an insulator; it has high dielectric breakdown, thermal stability up to 500℃ (932°F), resistance to corona discharge, and can maintain its electrical properties even in small particles.

[0005] However, the presence of mica in paper reduces its mechanical strength and cohesion. Combining mica paper with nonwoven fabrics, films, or glass fiber supports to improve mechanical properties is undesirable because it requires additional steps and various adhesives; and may increase paper thickness, reduce electrical properties, or create other problems, such as chemical compatibility with motor cooling fluids. Furthermore, the industry expects any improved paper to replace the very thin electrical insulating paper currently in use.

[0006] Furthermore, the manufacturing of electric motors and related equipment is now often automated, using machines to quickly and automatically insert electrical insulation material into slots and other motor areas. In these areas, the electrical insulation material may bend or detach due to wear between the motor components and the insulation. It has been found that in this process, such wear can cause mica-containing paper to shed mica particles, resulting in dust problems.

[0007] Therefore, there is a need for an aromatic polyamide paper containing mica that has sufficient strength and good resistance to particle shedding. Summary of the Invention

[0008] This invention relates to an aramid paper suitable for use as an electrical insulating material, comprising:

[0009] a) A first outer layer comprising 70 to 30 percent by weight of aramid flocs and 30 to 70 percent by weight of aramid fibers, the first outer layer being mica-free and having a first side and a second side;

[0010] b) An inner layer comprising 50 to 70 weight percent of an aromatic polyamide material and 30 to 50 weight percent of mica, the inner layer having a first side and a second side; and

[0011] c) A second outer layer comprising 70 to 30 percent by weight of aramid flocs and 30 to 70 percent by weight of aramid fibers, the second outer layer being mica-free and having a first side and a second side;

[0012] Wherein, the first outer layer has a first surface that is the first outer surface of the aramid paper, and the second surface of the first outer layer is co-extended with the first surface of the inner layer and is bonded together face-to-face only through the fibrous bodies in the first outer layer and the inner layer; and wherein, the first surface of the second outer layer is co-extended with the second surface of the inner layer and is bonded together face-to-face only through the fibrous bodies in the second outer layer and the inner layer, and the second surface of the second outer layer is the second outer surface of the aramid paper; the aramid paper has a total of 25 to 40 percent mica by weight. Detailed Implementation

[0013] This invention relates to an aramid paper suitable for use as an electrical insulating material, comprising: a first outer layer and a second outer layer, each of the first and second outer layers comprising 70 to 30 weight percent of aramid flocs and 30 to 70 weight percent of aramid fibrous material, each of the first and second outer layers further being mica-free (“mica-free”) and having a first side and a second side; an inner layer sandwiched between the first and second outer layers and comprising 50 to 70 weight percent of aramid material and 30 to 50 weight percent of mica, and also having a first side and a second side. The first side of the first outer layer is the first outer surface of the aramid paper, and the second side of the first outer layer is colinear with the first side of the inner layer and is bonded face-to-face only through the fibrous material in the first outer layer and the inner layer. Similarly, the first side of the second outer layer is colinear with the second side of the inner layer and is bonded face-to-face only through the fibrous material in the second outer layer and the inner layer, the second side of the second outer layer being 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 has a total of 25 to 40 percent mica by weight.

[0014] The mica-free outer layer, the mica-containing inner layer, and the mica-free outer layer are bonded together only by aramid slivers present on the faces of each layer. These slivers are uniformly distributed in each of these layers, with a portion of them present on the surface of each layer; that is, slivers are present on both faces of each layer. Thus, when the two layers are compressed together under increased heat and pressure, the slivers can be used at the surfaces of each layer to create a tight contact between the two layers and to bond or fuse the faces of the two layers together. This compression or consolidation can be accomplished in a static hot press or in a set of calendering rolls forming the gap, in a process in which the sheet can be compressed or consolidated at a temperature at or above the glass transition temperature of the slivers. Using slivers in each layer to bond these layers together eliminates the need for additional adhesives or other bonding agents, which not only require separate steps to apply but may also be chemically incompatible with the motor cooling fluid and other fluids that the electrical insulation materials may be exposed to during use.

[0015] As used herein, aramid paper refers to a flat sheet made of layers or sheets of aramid material produced by a papermaking process. Representative apparatus and machines that can be used to produce these layers or sheets include continuous processing equipment, such as, but not limited to, two-wire or inclined wire paper machines, or batch processing equipment, such as those used to manually produce paper in a hand-sheet mold containing a forming screen. Specifically, the aramid paper is a flat sheet comprising mica-containing aramid layers having first and second opposing flat surfaces, each of which is attached to a mica-free aramid layer. Thus, the aramid paper can consist of a mica-free aramid outer layer, a mica-containing aramid inner layer, and a mica-free aramid outer layer (attached together in this order).

[0016] Based on the total weight of mica in the total weight of the aramid paper, an aramid paper composed of a mica-free aramid layer, a mica-containing aramid layer, and a mica-free aramid layer (attached together in this order) has a total mica content of 25 to 40 wt%. It is believed that if the amount of mica in the aramid paper is less than 25 wt%, the measured partial discharge initiation voltage (PDIV) of the aramid paper will be too low for many applications. The partial discharge initiation voltage (PDIV) of paper is the voltage level at which partial dielectric breakdown begins in the paper. Therefore, the PDIV and the PDIV / mil are important characteristics because higher values ​​are desirable and lower values ​​mean that the insulation material may deteriorate rapidly during use, which may lead to lower insulation life and / or insulation failure. Similarly, if the amount of mica in the aramid paper is greater than 40 wt%, the mechanical properties of the paper may deteriorate to the point that the aramid material is unsuitable for many desired applications.

[0017] In some other embodiments, based on the total weight of mica in the total weight of the aramid paper, the aramid paper composed of a mica-free aramid layer, a mica-containing aramid layer, and a mica-free aramid layer (attached together in this order) has a total mica content of 30 to 40 percent by weight. In some still other embodiments, based on the total weight of mica in the total weight of the aramid paper, the aramid paper composed of a mica-free aramid layer, a mica-containing aramid layer, and a mica-free aramid layer (attached together in this order) has a total mica content of 35 to 40 percent by weight.

[0018] Aromatic polyamide materials may include aromatic polyamide flocs. As used herein, the term "flocs" refers to fibers cut to a short length and conventionally used in papermaking. Typically, flocs have a length of about 3 to about 20 mm. A preferred length is about 3 to about 7 mm. Flocs are typically produced by cutting continuous fibers to the desired length using methods well known in the art.

[0019] As used herein, the term "aromatic polyamide" means an aromatic polyamide in which at least 85% of the amide (-CONH-) bonds are directly attached to two aromatic rings. Optionally, additives may be used with the aromatic polyamide and may be dispersed throughout the polymer structure. It has been found that up to about 10% by weight of other polymeric materials can be blended with aromatic polyamides. It has also been found that copolymers of aromatic polyamides can be used with up to about 10% other diamines replacing the diamines of the aromatic polyamides or up to about 10% other diacyl chlorides replacing the diacyl chlorides of the aromatic polyamides.

[0020] Preferred aromatic polyamides are meta-aromatic polyamides. An aromatic polyamide polymer is considered meta-aromatic when two rings or groups are oriented meta-positionally relative to each other along the molecular chain. A preferred meta-aromatic polyamide is poly(m-phenylene isophthalamide) (MPD-I). U.S. Patent Nos. 3,063,966; 3,227,793; 3,287,324; 3,414,645; and 5,667,743 describe useful methods for preparing aromatic polyamide fibers that can be used to prepare aromatic polyamide flocs.

[0021] Aromatic polyamide materials can include aromatic polyamide fibrous strands. As used herein, the term "fibrous strand" refers to very small, non-granular, fibrous, or film-like particles, wherein at least one of its three dimensions is on the order of magnitude smaller than the largest dimension. These particles are prepared by precipitating a solution of polymeric material under high shear using a non-solvent-based method. Aromatic polyamide fibrous strands are non-granular, film-like particles of aromatic polyamides having a melting or decomposition point above 320°C. Preferred aromatic polyamide fibrous strands are meta-aromatic polyamide fibrous strands, and particularly preferred are strands made of meta-aromatic polyamide poly(m-isophthalamide) (MPD-I).

[0022] The fibrous material typically has a maximum length dimension ranging from about 0.1 mm to about 1 mm, with an aspect ratio of about 5:1 to about 10:1. The thickness dimension is on the order of a fraction of a micrometer, for example, from about 0.1 micrometer to about 1.0 micrometer. Although not required, it is preferred to incorporate the aromatic polyamide fibrous material into these layers while the fibrous material is in a never-dried state.

[0023] Aromatic polyamide paper has a first outer layer or sheet comprising 70 to 30 weight percent of aromatic polyamide flocs and 30 to 70 weight percent of aromatic polyamide fibrous strands, the first outer layer being mica-free and having a first side and a second side. Similarly, aromatic polyamide paper has a second outer layer or sheet comprising 70 to 30 weight percent of aromatic polyamide flocs and 30 to 70 weight percent of aromatic polyamide fibrous strands, the second outer layer being mica-free and having a first side and a second side. It is believed that these weight ranges will produce the best mechanical properties for the final aromatic polyamide paper. Mechanical strength is required so that the aromatic polyamide paper will not bend or detach during automatic insertion equipment (such as slot liner in an electric motor). Additionally, these weight ranges are considered necessary to provide the abrasion resistance expected during the paper's use. The first and second outer layers work synergistically in aramid paper, not only eliminating mica shedding by covering the paper with mica-containing layers or sheets, but also preventing the paper from being damaged during use by providing mechanical stability, which will release mica particles.

[0024] In some embodiments, the first and second outer layers of the aramid paper each comprise 60 to 40 weight percent of aramid flocs and 40 to 60 weight percent of aramid fibrous strands. In some preferred embodiments, the first and second outer layers of the aramid paper each comprise 50 to 40 weight percent of aramid flocs and 50 to 60 weight percent of aramid fibrous strands. In some embodiments, the first and second outer layers of the aramid paper have the same composition. In still other embodiments, the first and second outer layers of the aramid paper consist only of a mixture of aramid flocs and aramid fibrous strands.

[0025] The term "layer" preferably refers to a thin, planar material formed on a paper machine in the form of a forming wire. In most papermaking processes, an aqueous dispersion containing a desired paper composition is supplied to a screen, where the solid material in the dispersion forms a wet-laid wire, or sometimes referred to as a waterleaf, and water is removed by gravity, vacuum, and / or pressure. The wet-laid wire, after drying, becomes a "forming wire" as used herein. In the art, this forming wire is sometimes also referred to as a "sheet" or dry waterleaf. Therefore, as used herein, the term "layer" preferably refers to a forming wire, a sheet, or dry waterleaf, and these terms are used interchangeably. Furthermore, as used herein, the term "face" refers to either of the two principal surfaces of the layer (i.e., one side or the other side of the layer).

[0026] In one embodiment, the thickness of each of the first and second outer layers is 0.001 to 0.003 inches (1 to 3 mils); and in a preferred embodiment, the thickness of each of the first and second outer layers is 0.0015 to 0.002 inches (1.5 to 2 mils).

[0027] The aramid paper also has an inner layer or sheet comprising 50 to 70 weight percent of aramid material and 30 to 50 weight percent of 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 of aramid flocs and more than 50 weight percent of aramid fibrous strands. In some specific embodiments, the aramid material of the inner layer is a combination of 15 to 45 weight percent of flocs and 55 to 85 weight percent of fibrous strands.

[0028] Mica is typically used in sheet form and can be of various types, such as muscovite or phlogopite or blends thereof; however, muscovite-type mica is preferred.

[0029] It is believed that if the inner layer containing mica has more than 50% mica by weight, the properties of the final laminated structure will be negatively affected. First, on a constant weight basis, the tensile strength of the layer decreases with increasing mica content in the layer. Second, it is believed that having 50% or less mica by weight in the inner layer hinders mica bridging within the layer, promoting mica distribution throughout the layer. Furthermore, it is believed that if the inner layer containing mica has less than 30% mica by weight, there is insufficient mica in the layer to provide the desired electrical properties.

[0030] In one embodiment, the thickness of the mica-containing inner layer is 0.002 to 0.010 inches (2 to 10 mils); and in a preferred embodiment, the thickness of the inner layer is 0.004 to 0.006 inches (4 to 6 mils).

[0031] 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 co-linear with the first surface of the inner layer and is bonded together face-to-face only through the fibrous strands in the first outer layer and the inner layer. Similarly, the first surface of the second outer layer is co-linear with the second surface of the inner layer and is bonded together face-to-face only through the fibrous strands in the second outer layer and the inner layer, and the second surface of the second outer layer is the second outer surface of the aramid paper.

[0032] The phrase “extending with and facing each other” preferably means that each outer and inner layer has the same planar boundary; the edges of the outer and inner layers are the same, and the extension of any outer layer does not exceed the edge of the inner layer, or vice versa.

[0033] The phrase "face-to-face bonding" preferably means that each opposite face of the inner layer is bonded to one face of each of the outer layers. In some embodiments, these faces are uniformly bonded, meaning that the attachment of each face of the inner layer to each face of the outer layer is substantially uniform across the entire plane. "Substantially uniform" means that the layers are bonded together without any visible gaps or visually detectable non-attached discrete areas in the attachment. In some embodiments, these faces are continuously bonded, meaning that the entire surface of each face of the inner layer is attached to the entire surface of one of the faces of the outer layer.

[0034] Aromatic polyamide paper can be prepared by combining two mica-free outer layers (or sheets) with a mica-containing inner layer (or sheet) using the preparation techniques and conditions described in British Patent GB 1,129,097; US Patent Application Publication 2010 / 0122769 and US Patent 4,481,060, in a batch manner comprising several steps, or more preferably in a continuous or semi-continuous manner.

[0035] In some embodiments, the total thickness of the aramid paper, consisting of a mica-free aramid layer, a mica-containing aramid layer, and a mica-free aramid layer (attached together in this order), is 0.004 to 0.016 inches (4 to 16 mils). In some embodiments, particularly suitable for use in many high-voltage applications (600-800 volts), the total thickness of the aramid paper is 0.007 to 0.012 inches (7 to 12 mils); preferably, the total thickness is 0.008 to 0.010 inches (8 to 10 mils). In some other embodiments, such as for applications requiring lower voltages, the total thickness of the aramid paper is 0.004 to 0.007 inches (4 to 7 mils).

[0036] Furthermore, provided that the total thickness of the aramid paper, consisting of a first mica-free aramid outer layer, a mica-containing aramid inner layer, and a second mica-free aramid outer layer (attached together in this order), is 0.004 to 0.016 inches (4 to 16 mils), and the thickness of the mica-containing inner layer is 0.002 to 0.010 inches (2 to 10 mils), the mica-containing aramid inner layer can be formed from two or more mica-containing aramid sublayer sheets or layers, preferably two or more identical mica-containing aramid sublayer sheets or layers. Similarly, provided that the total thickness of the aramid paper, consisting of a first mica-free aramid outer layer, a mica-containing aramid inner layer, and a second mica-free aramid outer layer (attached together in this order), is 0.004 to 0.016 inches (4 to 16 mils), and the thickness of each of the first and second mica-free aramid outer layers is 0.001 to 0.003 inches (1 to 3 mils), then each of the mica-free aramid outer layers may be formed from two or more mica-free aramid sublayer sheets or layers, preferably two or more identical mica-free aramid sublayer sheets or layers. Preferably, the aramid paper is made by combining all the individual layers and / or any sublayer sheets or layers (if present) and then bonding all the layers together at once.

[0037] Aromatic polyamide paper, composed of a mica-free aramid layer, a mica-containing aramid layer, and a mica-free aramid layer (attached together in this order), provides effective shielding of the relatively weaker mica-containing inner layer by using a durable, mica-free outer layer. This prevents damage to the inner layer and associated mica shedding during installation into various devices that use the aramid paper. The shedding of aramid paper can be simulated using a Taber abrader, which operates continuously on the paper surface for multiple revolutions, mimicking the wear and tear on surfaces in 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 undergoes a Taber abrasion measurement after 125 revolutions on each arm using a 1000g weight (using a Taber 5150 abrasive (wear tester) and H018 wheel from Taber Industries, Tonawanda, NY), the weight loss (in grams) of the aramid paper does not exceed that of mica-free aramid paper of the same thickness in a similar test. Therefore, aramid paper containing a considerable amount of mica (25 to 40% by weight) unexpectedly exhibits similar shedding properties to mica-free sheets.

[0038] Test methods

[0039] Thickness was measured using ASTM-D374 (2010).

[0040] Partial discharge initiation voltage (PDIV) was measured using ASTM-D1868(2020).

[0041] The partial discharge initiation voltage per thickness (PDIV / mil) is calculated by dividing the partial discharge initiation voltage (PDIV) of a specified instance by the thickness (mil).

[0042] Tensile strength was measured using ASTM-D828 (2010).

[0043] Taber abrasion was measured on a Taber 5150 grinder according to its instructions (https: / / www.taberindustries.com / taber-rotary-abraser). The setup for paper measurements used an H-18 grinding wheel and 1000g weights on each arm, running 125 revolutions per sample. The weight loss of the sample was calculated using the sample weight before and after testing (excluding any loose surface particles).

[0044] Example 1

[0045] The aromatic polyamide paper, consisting of two mica-free outer mesh layers and a single mica-containing inner mesh layer, is prepared as follows.

[0046] To prepare mica-free layers or sheets, a mixture of 55 wt% poly(m-phenylene isophthalamide) (MPD-I) slivers and 45 wt% crystalline MPD-I flocs is prepared in water to form an aqueous dispersion suitable for use as a furnish. The MPD-I slivers are prepared as generally described in U.S. Patent No. 3,756,908, and the MPD-I flocs have a linear density of 0.22 tex and a length of 0.64 cm. The dispersion is supplied to the headbox of a fourdrinier paper machine and forms a wet-laid wire or unsized paper. The wet-laid wire is then dried to form an unconsolidated, mica-free aramid forming wire or layer with a thickness of 2 mils (0.002 inches). The speed of the paper machine is then reduced to prepare a similar forming wire or layer with a thickness of 3 mils (0.003 inches).

[0047] To prepare mica-containing layers or sheets, a mixture of 48 wt% muscovite-type mica, 37 wt% (MPD-I) filaments, and 15 wt% crystalline MPD-I flocculents is prepared in water to form a mica-containing aqueous dispersion suitable for use as a papermaking ingredient. The MPD-I filaments and MPD-I flocculents are the same as those used in mica-free layers. The mica-containing dispersion is supplied to the headbox of a fourdrinier paper machine and a wet-laid wire is formed. The wet-laid wire is then dried to form an unconsolidated mica-containing aramid forming wire or layer with a thickness of 3 mils (0.002 inches). The paper machine speed is then slowed down to prepare similar mica-containing forming wires or layers with thicknesses of 4 mils (0.004 inches), 6 mils (0.006 inches), 8 mils (0.008 inches), and 9 mils (0.009 inches).

[0048] To illustrate the possibility of detachment, samples of aramid paper were prepared, consisting of two mica-free outer mesh layers and a single mica-containing inner mesh layer sandwiched between these two outer layers. Specifically, the aramid paper was prepared by manually stacking the layers and bonding the three layers together in a static hot press operating at a surface temperature of 280°C, and then pressing these layers together under a pressure of 500 psi.

[0049] The properties of the obtained three-layer aramid paper are summarized in Item 1 of Table 1. Table 1 also shows three comparative samples, A, B, and C. Sample A is a two-layer paper with only two 3-mil mica-free mesh layers and no mica-containing mesh layers. Sample B is a three-layer paper with three 3-mil mica-free mesh layers and no mica-containing mesh layers. Sample C is a paper prepared using only one thick forming mesh or layer containing mica; no mica-free layers are present.

[0050] Samples were submitted for the Tiber abrasion test to determine the possible amount of material detached from the paper. After 125 revolutions, the sample was shaken to remove any loose surface particles. The sample was then weighed and compared to its pre-test weight to determine the grams of material lost from the paper during the 125 revolutions.

[0051] As shown, the contrasting aramid paper samples A and B, which lack mica, exhibited very little particulate matter, with 0.19 and 0.09 grams respectively; and due to the absence of mica, this is clearly an aramid fiber material. The aramid paper sample of the present invention in Item 1 has a similar particulate matter content of 0.15, indicating that virtually no mica shed from the paper. Contrasting aramid paper samples C and D demonstrate that a significant amount of particulate matter shed from mica-containing paper without an outer protective layer.

[0052] Table 1

[0053]

[0054]

[0055] Example 2

[0056] Various samples of aramid paper were prepared 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 them. Samples of aramid paper were prepared by manually stacking the layers and bonding the three layers together in a static hot press operating at a surface temperature of 280°C, and pressing these layers together under a pressure of 500 psi, as in Example 1. Specific samples and properties of the resulting three-layer paper are summarized in Table 1. For illustrative purposes, data from sample C of Example 1, which was prepared using only a thick mica-containing layer and had no mica-free layer, are also shown in Table 2.

[0057] Table 2

[0058]

[0059] Example 3

[0060] Samples of aramid paper were prepared as described above, having the composition from item 3 of Table 2, consisting of two mica-free outer layers and a single mica-containing inner layer sandwiched between these two outer layers; however, the individual layers were supplied from a single roll and combined and then bonded together in a continuous process in the roll gap between hot calendering rolls operated at a surface temperature of 280°C and pressed together at a pressure of 1300 lbs per linear inch to bond the three layers to a thickness of approximately 0.0102 inches (10.2 mils). The aramid paper has a PDIV of 1327 volts, equivalent to 130 volts per mil, and a tensile strength of 98.6 lbf.

[0061] Reference Example

[0062] This example illustrates the potential negative impact of adding mica on the mechanical properties of aramid paper sheets. Single-layer mica-free and mica-containing aramid handmade paper were prepared to demonstrate the effect of mica addition on sheet properties. Aqueous dispersions were prepared as described in Example 1, and individual handmade paper samples were then prepared using a handmade paper mold. Each aqueous dispersion was poured into a 21×21cm handmade paper mold with 8 liters of water to form five wet-laid papers: R-0, R-20, R-30, R-40, and R-50, where item R-0 contained no mica. The handmade paper was then placed individually between two sheets of blotting paper, hand-pressed with a rolling pin, and dried in a handmade paper dryer at 150°C for 10 minutes. As shown in Table 3, the final composition of the handmade paper varied between 0 and 50% by weight of mica, 37 to 65% by weight of MPD-I filaments, and 13 to 35% by weight of aromatic polyamide flocculants. The initial unconsolidated thickness of all handmade paper was a nominal 0.005 inches (5 mils) before pressing in a static press.

[0063] As shown in Table 3, although the addition of mica increases the partial discharge initiation voltage of handmade paper, it has a great negative impact on the mechanical properties of the sheet, most notably on the tensile strength of traditional mica-containing aramid handmade paper.

[0064] Table 3

[0065]

Claims

1. An aramid paper suitable for use as an electrical insulating material, comprising: a) A first outer layer comprising 70 to 30 percent by weight of aramid flocs and 30 to 70 percent by weight of aramid fibers, the first outer layer being mica-free and having a first side and a second side. b) An inner layer comprising 50 to 70 weight percent of an aromatic polyamide material and 30 to 50 weight percent of mica, the inner layer having a first side and a second side; and c) A second outer layer comprising 70 to 30 percent by weight of aramid flocs and 30 to 70 percent by weight of aramid fibers, the second outer layer being mica-free and having a first side and a second side; Wherein, 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 co-extended with the first surface of the inner layer and is bonded together face-to-face only through the fibrous strands in the first outer layer and the inner layer; and Wherein, the first surface of the second outer layer is co-extended with the second surface of the inner layer and is bonded together face to face only through the fiber bodies in the second outer layer and the inner layer, and the second surface of the second outer layer is the second outer surface of the aramid paper; The aramid paper contains a total of 25 to 40 weight percent mica; The term "co-extension" means that each outer and inner layer has the same planar boundary; the edges of the outer and inner layers are identical, and the extension of any outer layer will not exceed the edge of the inner layer, or vice versa.

2. The aramid paper as described in claim 1, wherein, The inner layer of aramid material is a combination of less than 50% by weight of aramid flocs and more than 50% by weight of aramid fibers.

3. The aramid paper of claim 2, wherein the aramid material of the inner layer is a combination of 15 to 45 percent by weight of flocculent material and 55 to 85 percent by weight of fibrous material.

4. The aramid paper according to any one of claims 1 to 3, wherein, The thickness of each of the first outer layer and the second outer layer is 0.001 to 0.003 inches.

5. The aramid paper as described in claim 4, wherein, The thickness of each of the first outer layer and the second outer layer is 0.0015 to 0.002 inches.

6. The aramid paper according to any one of claims 1 to 3, wherein, The thickness of the inner layer is 0.002 to 0.010 inches.

7. The aramid paper as described in claim 6, wherein, The thickness of the inner layer is 0.004 to 0.006 inches.

8. The aramid paper as described in any one of claims 1 to 3, wherein the total thickness is 0.004 to 0.016 inches.

9. The aramid paper of claim 8, wherein the total thickness is 0.007 to 0.012 inches.

10. The aramid paper of claim 9, wherein the total thickness is 0.008 to 0.010 inches.

11. The aramid paper of claim 8, wherein the total thickness is 0.004 to 0.007 inches.

12. The aramid paper according to any one of claims 1 to 3, wherein, When the paper was subjected to a Taber abrasion measurement after 125 revolutions with a 1000 g weight on each arm, the weight loss of the paper in grams did not exceed the weight loss of a similar test of mica-free aramid paper of the same thickness.

13. The aramid paper according to any one of claims 1 to 3, wherein the sliver has a maximum length dimension in the range of 0.1 mm to 1 mm, an aspect ratio of 5:1 to 10:1, and a thickness dimension of 0.1 micrometer to 1.0 micrometer.

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