Reconstituted Mica Materials, Reconstituted Mica Prepreg Materials, Reconstituted Mica Products and Insulated Coils

This invention relates to reconstituted mica materials, reconstituted mica prepreg materials, reconstituted mica products and insulated coils.

As reconstituted mica, there has generally been used so-called fired reconstituted mica produced by heating muscovite to about 800.degree. C. for dehydration, disintegrating expanded muscovite with stirring in water or by a jet stream of water to give a slurry of mica flakes or scales having a particle size of as small as about 0.2 to 1.0 mm, and sheet forming by using the resulting slurry containing mica flakes to give mica foils. There has also been used so-called soft reconstituted mica produced by disintegrating phlogopite in water to give a slurry of mica flakes having a particle size of as small as about 0.2 to 2.0 mm, and sheet forming by using said slurry to give mica foils.

Fired mica cannot be disintegrated industrially to thin flakes of 5 to 10 mm square. Moreover, since fine cracks are produced in mica flakes or scales after leaving the water of crystallization of mica, strength of mica scales is low and an effect as a reinforcing material is small. Further, fired reconstituted mica has a density of as low as 1.5 to 1.5 g/cm.sup.3 and a large number of voids in the mica foils, so that a large amount of thermosetting resin composition is necessary for giving, for example, prepreg materials, which results in giving products having a larger resin content with poor mechanical and electrical properties when molded with heating under pressure. In order to give heat resistant reconstituted mica, when a large amount of heat resistant thermosetting resin composition is used, there takes place lowering in heat resistance or remarkable deterioration in properties in a long period of time.

On one hand, phlogopite is slightly superior to muscovite in heat resistance but inferior to muscovite in electrical properties, so that it cannot be used for insulating high voltage machines and instruments such as a dynamo.

On the other hand, in the case of disintegrated unfired mica particles, it is possible to lower the amount of binder and the like at the time of processing them to reconstituted mica products compared with disintegrated fired reconstituted mica particles, but they are insufficient in mechanical and electrical properties compared with splitting mica products.

In order to improve the above-mentioned defects, unfired reconstituted mica under a trade name of Micanite II has been developed by U.S. SAMICA Corp. Micanite II is produced by disintegrating unfired muscovite flakes in water by using ultrasonic energy. Its particle size distribution is listed in Table 1 together with the particle size distribution of conventionally used reconstituted mica.

Micanite II is unfired reconstituted mica but has tensile strength of as strong as 0.7 to 1.3 kgf/mm.sup.2 without using a binder and the resulting mica foil has a density of as high as 1.6 to 1.7 g/cm.sup.3. But Micanite II has some defects caused by the particle size distribution as shown in Table 1. That is, since larger particles are used in Micanite II, reinforcing effect by the mica particles is great, which results in improving mechanical properties of processed mica products, but, on the other hand, since no closest packing can be attained by using larger mica particles, deterioration of properties, particularly electrical properties after a long period of time is more remarkable than splitting mica products.

Splitting mica products mean mica cleaved or delaminated by hand and having a diameter of about 50 mm or more and 30 to 50 .mu.m thick. Since the size of this mica is so large that it cannot be sheet formed by using a wet type paper machine unlike reconstituted mica and is sheet formed by hand or using a semi-automatic machine in practical production. Therefore, the production cost of splitting mica products becomes higher and finally the splitting mica products are more expensive than the reconstituted mica products because the former is not suitable for a large scale production by using a machine, and the like. Properties of the splitting mica products are excellent even after long time deterioration, since larger scales than a conventional reconstituted mica are used and the larger scales function as insulator even after the deterioration of binder.

In order to apply features of both the splitting mica products and the reconstituted mica products, combination of the both is proposed, for example, in Japanese patent appln. Kokai (Laid-Open) No. 58500/78, wherein both sides of delaminated mica sheet are bound with reconstituted mica sheets. The resulting product is improved in the accuracy of the thickness compared with the splitting mica product but rather lowered in properties after deterioration for a long period of time compared with the laminated mica product alone.

Improvement of the defects mentioned above has long been desired.

In the case of insulated electrical coils, they are produced by a process comprising winding splitting mica tape around a conductor, impregnating the wound tape with a thermosetting resin composition such as an epoxy resin composition, an unsaturated polyester resin under vacuum, and curing the resin, a so-called prepreg process comprising winding fired mica prepreg tape around a conductor, and molding with heating under pressure, or the like. The prepreg process is superior in initial electrical properties but not so good in other properties, while the vacuum impregnation process using splitting mica tape is superior in initial mechanical properties and electrical properties after deterioration by voltage endurance test but not so good in other properties.

When reconstituted mica is used, there has been no insulated coil having the same or higher properties compared with the case of using the splitting mica. An insulated coil having the same electrical properties as in the case of using the splitting mica is disclosed in Japanese patent appln Kokoku (Post-Exam publn) No. 20264/75. But the process disclosed therein has defects in that there are necessary complicated procedures such as wetting raw mica blocks with an aqueous solution of hydrofluoric acid or hydrogen chloride, and also necessary for treating waste water. Further the resulting insulated coil is not so sufficient in mechanical properties since fired mica is used.

It is an object of this invention to provide reconstituted mica materials improved in the defects mentioned above. It is another object of this invention to provide reconstituted mica prepreg materials and reconstituted mica products having improved properties by using said reconstituted mica materials. It is a further object of this invention to provide insulated coils having excellent electrical and mechanical properties overcoming the defects mentioned above.

This invention provides a reconstituted mica material produced by sheet forming a slurry containing mica flakes prepared by disintegrating unfired mica and comprising:

(i) 30 to 70 parts by weight of mica flakes having a particle size of 1.0 mm or more and an aspect ratio of 150 or more,

(ii) 20 to 40 parts by weight of mica flakes having a particle size of 0.25 mm or more and less than 1.0 mm, and

(iii) 10 to 30 parts by weight of mica flakes having a particle size of less than 0.25 mm, and if necessary, adhering the resulting mica sheet to a backing material.

This invention also provides a reconstituted mica prepreg material produced by sheet forming a slurry containing mica flakes prepared by disintegrating unfired mica and comprising:

(i) 30 to 70 parts by weight of mica flakes having a particle size of 1.0 mm or more and an aspect ratio of 150 or more,

(ii) 20 to 40 parts by weight of mica flakes having a particle size of 0.25 mm or more and less than 1.0 mm, and

(iii) 10 to 30 parts by weight of mica flakes having a particle size of less than 0.25 mm, to give a reconstituted mica material,

impregnating or coating the reconstituted mica material with a thermosetting resin composition, and if necessary bound together with a backing material, and

partly curing the impregnated or coated resin.

This invention further provides a reconstituted mica product produced by:

sheet forming a slurry containing mica flakes prepared by disintegrating unfired mica and comprising:

(i) 30 to 70 parts by weight of mica flakes having a particle size of 1.0 mm or more and an aspect ratio of 150 or more,

(ii) 20 to 40 parts by weight of mica flakes having a particle size of 0.25 mm or more and less than 1.0 mm, and

(iii) 10 to 30 parts by weight of mica flakes having a particle size of less than 0.25 mm, to give a reconstituted mica material,

impregnating or coating the reconstituted mica material with a thermosetting resin composition or an inorganic composition, and

molding the resulting material with heating under pressure.

This invention still further provides an insulated electrical coil comprising an electrical conductor and an insulating layer wrapped around said conductor, said insulating layer comprises

(a) 60 to 85 parts by weight of reconstituted mica material produced by sheet forming a slurry containing mica flakes prepared by disintegrating unfired mica and comprising (i) 30 to 70 parts by weight of mica flakes having a particle size of 1.0 mm or more and an aspect ratio of 150 or more, (ii) 20 to 40 parts by weight of mica flakes having a particle size of 0.25 mm or more and less than 1.0 mm, and (iii) 10 to 30 parts by weight of mica flakes having a particle size of less than 0.25 mm,

(b) 15 to 30 parts by weight of a thermosetting resin composition, and

(c) 15 parts by weight or less of a backing material, the total weight of (a), (b) and (c) being 100 parts by weight.

The insulating layer can be formed by a conventional process, for example, a prepreg process, a vacuum impregnation process, or the like by using various forming conditions.

In this invention, the aspect ratio is defined as follows: ##EQU1##

Particle size distribution is measured by classifying mica flakes by a wet process using sieve analysis (Tyler mesh) and weighing after dried for calculation.

The process of disintegrating unfired mica to give mica flakes having the desired particle sizes and aspect ratios is disclosed, for example, in Japanese patent appln Kokoku (Post-Exam publn) No. 8899/79, Japanese patent appln Kokai (Laid-Open) No. 39984/78, and the like and in U.S. Pat. No. 4,165,839. The processes disclosed in these publications are particularly preferable for easily giving mica flakes having lange aspect ratios and desired particle size distribution, since a disintegration apparatus and a classification apparatus are designed as one apparatus in these publications.

In order to form mica sheet, there should be used a slurry containing the following mica flakes prepared by disintegrating unfired mica:

(i) Mica flakes having a particle size of 1.0 mm or more and an aspect ratio of 150 or more in an amount of 30 to 70 parts by weight.

If the amount is less than 30 parts by weight, since a reinforcing effect of mica flakes is small, the resulting reconstituted mica products or insulated coils are lowered in mechanical properties. On the other hand, if the amount is more than 70 parts by weight, although the reinforcing effect is increased, electrical properties of the resulting reconstituted mica products and insulated coils are lowered because of larger voids produced in the main sheet.

Further, the aspect ratio should be 150 or more. If the aspect ratio is less than 150, mechanical properties and electrical properties of the resulting reconstituted mica products and insulated coils are undesirably lowered. (ii) Mica flakes having a particle size of 0.25 mm or more and less than 1.0 mm in an amount of 20 to 40 parts by weight.

If the amount is less than 20 parts by weight, the closest packing of mica flakes in the resulting mica sheet cannot be obtained, which results in lowering in electrical properties, particularly in dielectric breakdown voltage of the resulting reconstituted mica products or insulated coils. On the other hand, if the amount is more than 40 parts by weight, mechanical properties of the resulting reconstituted mica products or insulated coils are lowered because of an increase in the proportion of mica flakes having smaller particle sizes. (iii) Mica flakes having a particle size of less than 0.25 mm in an amount of 10 to 30 parts by weight.

If the amount is less than 10 parts by weight, it is impossible to sufficiently fill spaces produced by using mica flakes having a particle size of 1.0 mm or more, which results in requiring a larger amount of binder for adhering the mica sheet to a backing material or a larger amount of thermosetting resin composition for filling spaces among mica flakes in the case of insulated coils or prepregs and lowering electrical properties due to deterioration after a long period of time. On the other hand, if the amount is more than 30 parts by weight, mechanical properties and electrical properties of the resulting reconstituted mica products or insulated coils are lowered because of an increase in the proportion of mica flakes having smaller particle sizes.

In this invention, the slurry for making mica sheet should contain the mica flakes (i), (ii) and (iii) mentioned above. Even if combining mica flakes having a particle size of less than 0.25 mm and those having a particle size of 1.0 mm or more, no mica sheet having the closest packing of mica flakes can be obtained, since the precipitation speeds of the former and the latter are different in 10 times or more and when sheet formed by using a wet type paper making machine, large mica flakes may be collected on one side and small mica flakes on the other side. Such a defect can only be removed by the use of the mica flake (ii) in the above-mentioned range.

The unfired mica includes mica not subjected to a firing treatment and mica obtained by removing organic components such as paper, wood, fibers and the like from scrap mica, cut mica, and the like by complete combustion in the air at a temperature from that not liberating the water of crystallization of mica (about 600.degree. C. or lower) to that liberate 1/2 or less of the water of crystallization of mica.

When fired mica is used in place of the unfired mica, mechanical properties, particularly flexural strength and flexural modulus, are lowered remarkably in the resulting reconstituted mica materials, reconstituted mica prepreg materials and reconstituted mica products.

In this invention, any conventional processes for making mica sheet from the slurry can be used. For example, a slurry containing 0.5 to 2% by weight of mica flakes can be sheet formed by using, for example, a Fourdrinier paper machine, a cylinder paper machine, or the like under conventional conditions.

If necessary, the resulting mica sheet is adhered to a backing material. As the backing materials, there can be used woven fabric, nonwoven fabric, film, and the like made from organic materials such as polyesters, polyamides, polypropylenes, or inorganic materials such as glass, alone or in combination, and if necessary, together with glass yarns, polyester fiber yarns, etc. In such a case, a conventional binder such as a thermosetting resin is used for binding the both materials.

The resulting reconstituted mica material (in the form of sheet) can be used for producing a reconstituted mica prepreg material by impregnating or coating it with a thermosetting resin composition, and if necessary bound together with a backing material, and partly curing the impregnated or coated resin.

As the thermosetting resin composition, there can be used an epoxy resin composition, an unsaturated polyester resin composition, a silicone resin composition, and the like including therein one or more curing agents, surface active agents, solvents, reactive solvents, and the like conventional additives.

The impregnation or coating of the thermosetting resin composition can be carried out by a conventional method, for example, by dissolving the thermosetting resin composition in a solvent such as methyl ethyl ketone, acetone, methanol, etc., and impregnating the resulting solution into the reconstituted mica material or coating it using a conventional coating machine such as a spray nozzle, a brush coating machine, etc. After the impregnation or coating, the solvent is removed by drying the resulting material using a drier with hot air, infrared rays, or the like.

The partial curing of the impregnated or coated resin can be carried out by a conventional method.

The same backing materials as mentioned above can also be used for producing the reconstituted mica prepreg material.

The reconstituted mica material (in the form of sheet) can also be used for producing a reconstituted mica product by impregnating or coating it with a thermosetting resin composition or an inorganic composition, and molding the resulting material with heating under pressure. As the thermosetting resin composition, those mentioned above for producing the prepreg can also be used. As the inorganic composition, there can be used compositions comprising aluminum phosphate, borosilicate glass, silicate glass, etc.

The same impregnation or coating method as mentioned above for producing the prepreg can also be used in this case.

The molding can be carried out with heating at a temperature sufficient for softening the thermosetting resin composition or higher and under pressure sufficient for making the resulting molded article into one piece and maintaining the pressure for a sufficient time for curing the softened thermosetting resin composition and shaping the product.

The reconstituted mica material (in the form of sheet) with or without a backing material can also be used for producing an insulated coil comprising an electrical conductor and an insulating layer wrapped around said conductor.

The insulating layer can be formed either by a so-called vacuum impregnation process or by a so-called prepreg process. There is no particular limit to the forming conditions.

More in detail, the reconstituted mica material, if necessary having a backing material, is wrapped around a conductor, impregnating the wrapped mica material with a thermosetting resin composition under reduced pressure or under pressure, and curing the thermosetting resin composition to give an insulated coil. Alternatively, a reconstituted mica prepreg material is formed first by impregnating the reconstituted mica material with a thermosetting resin composition, followed by partial curing of the resin, and then the resulting prepreg material is wrapped around a conductor and the resin is cured to give an insulated coil.

As the thermosetting resin composition and the backing material, those described above, e.g., for producing the reconstituted mica prepreg material, can also be used.

The insulating layer comprises (a) 60 to 85 parts by weight of the reconstituted mica material, (b) 15 to 30 parts by weight of the thermosetting resin composition and (c) 15 parts by weight or less of the backing material, the total weight being 100 parts by weight.

The reconstituted mica material is used in the insulating layer in an amount of 60 to 85 parts by weight. If the amount is less than 60 parts by weight, electrical properties such as dielectric strength of the insulated coil is lowered, while if the amount is more than 85 parts by weight, electrical properties and mechanical properties of the insulated coil is lowered remarkably due to much more spaces among the mica flakes.

The thermosetting resin composition is used in the insulating layer in an amount of 15 to 30 parts by weight. If the amount is less than 15 parts by weight, spaces are easily formed in the insulating layer to cause corona discharge and electrical properties of the insulated coil is lowered, and when stress such as bending, compression, or the like is given to the insulating layer, there takes place stress concentration, which results in lowering mechanical properties. On the other hand, if the amount is more than 30 parts by weight, not only mechanical properties of the insulated coil but also electrical properties after deterioration for a long period of time are lowered.

The backing material is used by binding it to the reconstituted mica material, if necessary, up to 15 parts by weight. If the amount is more than 15 parts by weight in the insulating layer, mechanical properties or electrical properties are lowered. For example, when woven fabric or nonwoven fabric of glass fiber is used as a backing material for the reconstituted mica material in an amount exceeding 15 parts by weight, the resulting insulated coil is lowered in electrical properties, but not in mechanical properties. When woven fabric, non-woven fabric or film of polyester, polyamide, or the like is used as a backing material for the reconstituted mica material in an amount exceeding 15 parts by weight, the resulting insulated coil is lowered in mechanical properties and electrical properties after deterioration for a long period of time. The same thing can be applied to the case of using the combination of the film mentioned above and glass, or the combination of the film mentioned above and yarns of polyester fiber, and the like.

This invention is illustrated by way of the following Examples, in which all parts and percents are by weight unless otherwise specified.

Reconstituted mica materials as listed in Table 2 were prepared by using slurries containing mica flakes in an amount of about 1% dispersed in water and a Fourdrinier paper machine for sheet forming. Table 2 also shows particle size distribution of mica flakes used in each material.

On the other hand, thermosetting resin compositions or an inorganic composition as listed in Table 3 were prepared.

The reconstituted mica material 1 listed in Table 2 was coated with the composition 1 listed in Table 3 as adhesive heated at 60.degree. C. in an amount of 100 g/m.sup.2 and bound to glass cloth (35 g/m.sup.2) as a backing material, followed by heating at 80.degree. C. for 1 hour to give partially cured reconstituted mica prepreg material. The resulting prepreg was cut to a tape 30 mm wide. The tape was wrapped around a conductor (made of copper) of 9.5 mm.times.36.5 mm with 1 m long 8 times with an overlap of the order of 50%. Then, the resulting wrapped conductor was heated at 100.degree. C. and pressed so as to flow the composition 1 out of the reconstituted mica prepreg material, while curing the composition 1 at 170.degree. C. for 3 hours to give an insulated coil having an insulating layer 3 mm thick. In the same manner, there were produced a total of four insulated coils, two of which were used for a test under ordinary state and the rest of which were used for a test after thermal deterioration. Flexural strength of a coil was measured according to a so-called four point method (outer span 550 mm, inner span 250 mm). Dielectric breakdown test was conducted by raising a voltage at a speed of 2 kV/sec. Dielectric breakdown strength after deterioration was measured after thermal deterioration at 130.degree. C. for 1000 hours.

The results are as shown in Table 4, in which average values are employed.

Using the material 2 listed in Table 2, partially cured reconstituted mica prepreg material was prepared in the same manner as described in Example 1. Insulated coils were produced by using the resulting prepreg and their properties were evaluated, in the same manner as described in Example 1.

The results are as shown in Table 4, in which average values are employed.

Using the material 3 listed in Table 2, partially cured reconstituted mica prepreg material was prepared in the same manner as described in Example 1. In the same manner as described in Example 1, insulated coils were produced by using the resulting prepreg and their properties were evaluated.

The results are as shown in Table 4, in which average values are employed.

Using the material 5 listed in Table 2, partially cured reconstituted mica prepreg material was prepared in the same manner as described in Example 1. Insulated coils were produced by using the resulting prepreg and their properties were evaluated, in the same manner as described in Example 1.

The results are as shown in Table 4, in which average values are employed.

The reconstituted mica material 1 listed in Table 2 was coated with the composition 2 listed in Table 3 as adhesive in an amount of 174 g/m.sup.2 (nonvolatile content 10%) and dried at 80.degree. C. for 30 minutes to give partially cured prepreg mica sheets. Eight plys of the resulting mica sheets were piled, placed in a press heated at 150.degree. C. and pressed under a pressure of 20 kgf/cm.sup.2 for 20 minutes, followed by heating at 200.degree. C. under the same pressure for 2 hours to give a heat resistant mica product.

Properties of the resulting mica product were tested as follows:

Flexural strength was measured by a so-called 3 point method (span 50 mm) at a test speed of 1 mm/min. Weight loss on heating was measured by heating at 500.degree. C. for 2 hours. Resistance to smoking was measured by using a sample prepared by wrapping a nichrome wire (13.2 .OMEGA./m) 17 times around a test piece of 60.times.140 mm and sending an alternating current of 115-120 V with 3.5 A for 5 minutes.

The results are as shown in Table 5.

The reconstituted mica material 2 listed in Table 2 was coated with the composition 2 listed in Table 3 as adhesive in an amount of 198 g/m.sup.2 (nonvolatile content 10%) and dried at 80.degree. C. for 30 minutes to give partially cured prepreg mica sheets. Using the resulting mica sheets, a heat resistant mica product was produced in the same manner as described in Example 3.

Properties of the mica product were evaluated in the same manner as described in Example 3 and listed in Table 5.

The reconstituted mica material 4 listed in Table 2 was coated with the composition 2 listed in Table 3 as adhesive in an amount of 198 g/m.sup.2 (nonvolatile content 10%) and dried at 80.degree. C. for 30 minutes to give partially cured prepreg mica sheets. Using the resulting mica sheets, a heat resistant mica product was produced in the same manner as described in Example 3.

Properties of the mica product were evaluated in the same manner as described in Example 3 and listed in Table 5.

The reconstituted mica material 5 listed in Table 2 was coated with the composition 2 listed in Table 3 as adhesive in an amount of 247 g/m.sup.2 (nonvolatile content 10%) and dried at 80.degree. C. for 30 minutes to give partially cured prepreg mica sheets. Using the resulting mica sheets, a heat resistant mica product was produced in the same manner as described in Example 3.

Properties of the mica product were evaluated in the same manner as described in Example 3 and listed in Table 5.

On the reconstituted mica material 1 listed in Table 2, glass cloth (35 g/m.sup.2) as a backing material was piled. The composition 3 listed in Table 3 as adhesive was coated on the glass cloth in an amount of 75 g/m.sup.2 (nonvolatile content 20%) and dried at 100.degree. C. for 30 minutes. The resulting material was cut to a tape 30 mm wide. The tape was wrapped around a conductor (made of copper) of 9.5 mm.times.36.5 mm with 1.0 m long 8 times with an overlap of the order of 50%. Then, deaeration was conducted at 100.degree. C. and 0.1 mmHg for 2 hours, followed by impregnation in vacuum with the composition 1 listed in Table 3 heated at 80.degree. C. The pressure was then increased to normal pressures while dipping the coil in the composition 1 and the coil was taken out after 1 hour and covered with Lumirror film 2 mil thick (manufactured by Toray Industries, Inc.) so as to prevent the coil from dropping out of the composition 1. After cured at 110.degree. C. for 4 hours and 200.degree. C. for 1 hour, an insulating layer 3 mm thick was formed. In the same manner, there were produced a total of four insulated coils, two of which were used for a test under ordinary state and the rest of which were used for a test after thermal deterioration.

Properties of the insulated coils were evaluated in the same manner as described in Example 1. The results are shown in Table 6 with average values.

Using the material 2 listed in Table 2, reconstituted mica tape was produced in the same manner as described in Example 5 and insulated coils were produced in the same manner as described in Example 5.

Properties of the insulated coils were evaluated in the same manner as described in Example 1. The results are shown in Table 6 with average values.

Using the material 3 listed in Table 2, reconstituted mica tape was produced in the same manner as described in Example 5 and insulated coils were produced in the same manner as described in Example 5.

Properties of the insulated coils were evaluated in the same manner as described in Example 1. The results are shown in Table 6 with average values.

Using the material 5 listed in Table 2, reconstituted mica tape was produced in the same manner as described in Example 5 and insulated coils were produced in the same manner as described in Example 5.

Properties of the insulated coils were evaluated in the same manner as described in Example 1. The results are shown in Table 6 with average values.

The reconstituted mica material 1 listed in Table 2 was coated with the composition 4 listed in Table 3 as adhesive in an amount of 247 g/m.sup.2 and dried at 120.degree. C. for 5 minutes to give partially cured reconstituted mica sheets. Three plys of the resulting mica sheets were piled and cured by heating at 300.degree. C. under a pressure of 100 kgf/cm.sup.2 for 1 hour to give a mica laminate 0.3 mm thick.

Properties of the mica laminate were shown in Table 7.

The material 2 listed in Table 2 was coated with the composition 4 listed in Table 3 in an amount of 247 g/m.sup.2 and a mica laminate was produced in the same manner as described in Example 7.

Properties of the mica laminate were shown in Table 7.

The material 3 listed in Table 2 was coated with the composition 4 listed in Table 3 in an amount of 247 g/m.sup.2 and a mica laminate was produced in the same manner as described in Example 7.

Properties of the mica laminate were shown in Table 7.

The material 5 listed in Table 2 was coated with the composition 4 listed in Table 3 in an amount of 278 g/m.sup.2 and a mica laminate was produced in the same manner as described in Example 7.

Properties of the mica laminate were shown in Table 7.

As mentioned in detail in Examples, since the reconstituted mica materials of this invention use mica flakes having larger aspect ratios, the sheet strength is so strong as being possible to be taken up without using an adhesive. Further, reconstituted mica prepreg materials and reconstituted mica products produced by using the reconstituted mica materials are improved in mechanical properties since there are used mica flakes having a larger particle size and are also improved in electrical properties since the closest packing is obtained by filling spaces between mica flakes of larger particle sizes by those of smaller particle sizes. Therefore, compared with conventional reconstituted mica products (e.g., the materials 3 to 5 being used), the properties are improved, although the amount of the composition to be impregnated or coated is small. Particularly, as to the properties after deterioration for a long period of time, the properties higher than those of the splitting mica products can be obtained.

Reconstituted mica materials (materials 6 to 10) having a basis weight of 200 g/m.sup.2 as listed in Table 8 were prepared by using slurries containing mica flakes in an amount of 0.5% and a Fourdrinier paper machine for sheet forming at a rate of 5 m/min at 80.degree.-90.degree. C. Table 8 also shows particle size distribution of mica flakes used in each material.

On the other hand, an epoxy resin composition as listed in Table 9 was prepared.

The material 6 (reconstituted mica material) listed in Table 8 was coated with the epoxy resin composition listed in Table 9 heated at 60.degree. C. in an amount of 100 g/m.sup.2 and bound to glass cloth (35 g/m.sup.2) as a backing material, followed by heating at 80.degree. C. for 1 hour to give partially cured reconstituted mica prepreg material. The resulting prepreg was cut to a prepreg tape 30 mm wide. The tape was wrapped around a conductor (made of copper) of 9.5 mm crosswise, 36.5 mm lengthwise and 1000 mm long 8 times with an overlap of the order of 50%. Then, the resulting wrapped conductor was heated at 100.degree. C. and pressed so as to flow the epoxy resin composition out of the mica prepreg material, while curing the epoxy resin composition at 170.degree. C. for 3 hours to give an insulated coil having an insulating layer about 3 mm thick. In the same manner, there were produced a total of four insulated coils, two of which were used for a test under ordinary state and the rest of which were used for a test after thermal detrioration (130.degree. C. for 10.sup.3 hours). Dielectric breakdown test was conducted by raising a voltage at a speed of 2 kV/sec. Flexural strength was measured according to the four point method (outer span 550 mm, inner span 250 mm, test speed 5 mm/min). The amount of the epoxy resin composition (adhesive) was measured by heating the insulating layer at 600.degree. C. for 2 hours.

The results are shown in Table 10 with average values.

Using the material 7 listed in Table 8 and the epoxy resin composition listed in Table 9, prepreg tape was prepared in the same manner as described in Example 9. Four insulated coils were produced by using the resulting prepreg tape in the same manner as described in Example 9. Properties of the insulated coils were evaluated in the same manner as described in Example 9. The results are listed in Table 10 with average values.

Using the material 8 listed in Table 8 and the epoxy resin composition listed in Table 9, prepreg tape was prepared in the same manner as described in Example 9. Four insulated coils were produced by using the resulting prepreg tape in the same manner as described in Example 9. Properties of the insulated coils were evaluated in the same manner as described in example 9. The results are listed in Table 10 with average values.

Using the material 9 listed in Table 8 and the epoxy resin composition listed in Table 9, prepreg tape was prepared in the same manner as described in Example 9. Four insulated coils were produced by using the resulting prepreg tape in the same manner as described in Example 9. Properties of the insulated coils were evaluated in the same manner as described in example 9. The results are listed in Table 10 with average values.

Using the material 10 listed in Table 8 and the epoxy resin composition listed in Table 9, prepreg tape was prepared in the same manner as described in Example 9. Four insulated coils were produced by using the resulting prepreg tape in the same manner as described in Example 9. Properties of the insulated coils were evaluated in the same manner as described in Example 9. The results are listed in Table 10 with average values.

On the material 7 listed in Table 8, glass cloth (35 g/m.sup.2) as a backing materal was piled. A varnish prepared by dissolving the epoxy resin composition listed in Table 9 with methyl ethyl ketone and containing a nonvolatile content of 20% was coated on the glass cloth in an amount of 75 g/m.sup.2 (converted to nonvolatile content 15 g/m.sup.2) and dried at 100.degree. C. for 30 minutes. The resulting material was cut to a prepreg tape 30 mm wide. The prepreg tape was wrapped around the same conductor as used in Example 9, 8 times with an overlap of the order of 50%. Then, the resulting coil was dried at 100.degree. C. and 0.1 mmHg for 2 hours and impregnated with the epoxy resin composition listed in Table 9 heated at 80.degree. C. under the same pressure. The pressure was then increased to normal pressures while dipping the coil in the epoxy resin composition, and the coil was taken out after 1 hour and covered with Lumirror film 2 mil thick (manufactured by Toray Industries, Inc.) so as to prevent the coil from dropping out of the epoxy resin composition. After cured at 110.degree. C. for 4 hours and 170.degree. C. for 3 hours, an insulating layer 3 mm thick was formed.

Properties of the resulting four insulated coils were evaluated in the same manner as described in Example 9. The results are isted in Table 10 with average values.

Using the material 10 listed in Table 8, prepreg tape was prepared in same manne as described in Example 11. Four insulated coils were produced by using the resulting prepreg tape in the same manner as described in Example 11. Properties of the insulated coils were evaluated in the same manner as described in Example 9. The results are listed in Table 10 with average values.

The insulated coils of this invention have insulating layers of enhanced mechanical properties, since reconstituted mica materials formed from mica flakes having larger particle sizes and higher aspect ratios are used, and also have insulating layers of enhanced electrical properties since spaces between larger mica flakes can be filled with smaller mica flakes.

Further, when a reconstituted mica material wherein spaces between larger mica flakes are filled with smaller mica flakes is used, the amount of the thermosetting resin composition (used as adhesive) can be reduced. And even if the amount of the thermosetting resin composition is reduced, initial properties such as electrical and mechanical properties of the insulated coils are not lowered and still excellent in properties after deterioration for a long period of time (for example, properties after deterioration by voltage endurance test), which properties are the same or higher than those of insulated coils using splitting mica products.