Electric Insulating Paper And Its Production Process

Abstract

A process for the production of an electric insulating paper and the insulating paper thus produced, having superior characteristics such as a low dielectric constant, a low dielectric loss tangent, and a high dielectric strength, is disclosed. The process comprises the steps of preparing a graft-cellulose solution, or a mixed solution or suspension consisting of a synthetic polymer mixed into a cellulose solution or a graft-cellulose solution, solidifying the solution, mixed solution, or suspension into a regenerating substance such as air or a precipitant, forming the regenerated substance thus solidified into a pulplike substance adapted to be fabricated into a paper, and forming the pulplike substance only or the pulplike substance mixed with the ordinary beaten pulp into an insulating paper.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for the production of an electric insulating paper and also to the insulating paper itself, which has a far lower dielectric constant and dielectric loss tangent (tan .delta.), a higher dielectric strength, and superior oil resistance and heat resistance than conventional electric insulating papers.

2. Prior Art

Recently, the voltage rating of electric power cables has increased, for example, extrahigh-voltage electric power cables of 500kv. are now being employed, and furthermore, with the progress in atomic power generation, a cable of more than 750 kv. is going to be needed in the near future.

In electric power cables carrying such extrahigh voltages, electric insulating materials of far lower dielectric constants and dielectric loss tangents, of higher dielectric strengths, and of superior oil resistance and heat resistance, and having structures easily impregnated with insulating oil are required for their construction. Furthermore, the above-mentioned features of the insulating materials must be maintained for long periods. For instance, in an extrahigh voltage cable of 500 kv., it is considered essential that the dielectric constant be lower than 3.0 and the dielectric loss tangent (tan .delta.) be less than 0.1 percent (under the condition of 80.degree. C., oil filled, and with the application of 10 kv./mm.).

Heretofore, deionized insulating paper has been employed in a 275 kv. transmission cable. However, in a transmission cable of a voltage of 500 kv. or more, and when the cable is employed as a long distance line, even the dielectric constant and the dielectric loss tangent of the deionized insulating paper are still too large, resulting in the disadvantage that the transmission capacity of the cable is considerably decreased by use of such insulating paper.

Recently, various synthetic polymers having lower dielectric constants and dielectric loss tangents well adapted to be employed in the extrahigh-voltage transmission cables have been developed. For instance, polycarbonates or polyphenylene oxides employed in the form of film or tape have been suggested for use in the cables. However, such a material has the drawbacks of an inferior mechanical strength when it is employed in the form of a thin film of from 20 to 30 microns, although the impulse breakdown strength thereof is considerably high, and of abruptly decreasing its impulsive breakdown strength when it is employed in the form of thick film of more than 100 microns although the mechanical strength is improved. In the last case, the impulse breakdown strength is decreased to about 100 kv./mm. Furthermore, when the above described synthetic materials are employed in the cable, there is a great chance of causing crazing and cracking, and the materials have a tendency to swell under the action of the insulating oil, so that when used in the cable sufficient passages for the insulating oil are difficult to be maintained. For these reasons, the synthetic polymers or plastic films have not yet been employed practically in extrahigh-voltage transmission cables.

To eliminate the above-described drawbacks of the synthetic polymer or plastic film, a method of winding the plastic film interleavingly between sheets of paper on conductor or bonding the plastic film beforehand with a sheet of paper has also been proposed. However, with such a method, the above-described disadvantages of the plastic film, namely, the tendency for swelling or being dissolved in the insulating oil or of causing crazing and cracking cannot be fully eliminated. Furthermore, the dielectric strength of the composite material is comparatively low and the oil passages are also difficult to be maintained as in the case of the plastic tape.

Another attempt for eliminating the above-mentioned drawbacks of the plastic film and for utilizing joiningly the advantageous features of the synthetic polymers and those of paper, an insulating paper wherein fine particles, filaments, or fibers of the polymer are built into the paper together with the pulp has also been studied. However, difficulties have been experienced in this process due to the difference of specific gravities between the synthetic polymer and cellulose pulp and in its lowered mechanical strength. Since the mixing process of the synthetic polymer and cellulose is carried out in a macroscopic scale, various disadvantages found in the plastic film result in this mixed paper, and the dielectric loss tangent of the mixed paper is increased abruptly during its long working period, or the density and air tightness of the mixed paper tend to be low because of the difference in affinity of the synthetic polymer and the cellulose. This results in an inferior dielectric strength to the ordinary insulating paper. For this reason, this method has not yet been successfully employed in the production of an insulating paper to be employed in extrahigh-voltage cables.

Investigations into this problem from the point that, for the purpose of obtaining a superior electric insulating paper by combining the oil-resistant and stable nature of cellulose, the facility for oil immersion due to the porous structure of paper, and the low dielectric constant, the low dielectric loss tangent, the high dielectric strength, and the high heat resistance of the synthetic polymer, there is the necessity for coupling the component elements on a more microscopic molecular level, differing from the above-described macroscopic coupling, between the cellulose and the synthetic polymers. As a result, a successful production of an insulating paper employable in extrahigh-voltage power cables and having a low dielectric constant, a low dielectric loss tangent, a high dielectric strength, superior heat-resistance and oil-resistance nature, and a high mechanical strength has resulted.

SUMMARY OF THE INVENTION

The insulating paper according to the present invention is characterized in that the synthetic polymer and the cellulose are coupled or connected at a molecular level. More specifically, novel organic solvent capable of dissolving the cellulose is employed and the thus-obtained solution is added to another solution containing the synthetic polymer for blending the cellulose together with the synthetic polymer. By regeneratively solidifying the solution thus blended in a precipitant, a mixed product of the cellulose and the synthetic polymer can be obtained. When the product is beaten and made into a fibrillic substance, a pulplike substance can be obtained, and when this pulplike substance is added to the required amount of beaten pulp and made into paper, a product having a paperlike structure and a porous nature and wherein the synthetic polymer and the cellulose are mixed in a molecular level can be obtained.

Comparing the electric insulating paper thus obtained with the conventional electric insulating paper, the electric insulating paper of this invention has a far lower dielectric constant, a lower dielectric loss tangent, a high dielectric strength, and sufficient oil passages due to its porous nature than the conventional paper. Furthermore, the mechanical characteristics, such as tensile strength, elongation, tearing strength, and Young's coefficient, is equivalent or slightly higher than those of conventional insulating paper. In the insulating paper according to the present invention, since the synthetic polymer and the cellulose are mixed on a microscopic molecular level, the cracking and crazing caused in the conventional plastic films are not created therein, and the advantageous features of having a sufficient oil resistance, facility in making into paper in comparison with the conventional construction wherein particles or fibers of the synthetic polymer are mixed together, a high density and air tightness, and a superior dielectric strength are thereby achieved. Furthermore, in the novel insulating paper, the heat resistance of the synthetic polymer is utilized, so that a cable employing the insulating paper can endure higher temperatures than conventional insulating papers and can be used for a longer period than the conventional insulating paper.

Furthermore, also in accordance with the present invention, a suitable synthetic polymer which is insoluble in organic solvents but has a superior electric nature, for example, cross-linked polyethylene, polytetrafluoroethylene, or polypropylene, is added in the form of fine particles into the cellulose solution so that the synthetic polymer and the cellulose are blended together. The mixed solution may be solidified thereafter in a fibrous or granular configuration. In this way, the fine particles of the synthetic polymer are incorporated within the cellulose in such a manner that the synthetic polymer is enveloped by the cellulose, and the insulating paper, obtained from the fibrous or granular substance subsequently being made into paper together with the beaten pulp, has equivalent characteristics to that obtained by the above-described procedure wherein the synthetic polymer is blended in its liquid state. That is, the insulating paper thus obtained has a far superior heat resistance, oil resistance, and a more stable nature than the conventional insulating paper made of synthetic fibers or of synthetic polymer fibers.

Various methods other than those described herein for dissolving cellulose have been known,. However, most of the methods utilize water as the solvent, hence it is impossible to blend cellulose with the synthetic polymer. Furthermore, since most of the methods also use inorganic metallic salts in the procedure, the remaining metallic ions in the regenerated cellulose deleteriously affect the dielectric loss tangent even if the cellulose is blended with the synthetic polymer.

In contrast to the above methods, the method according to the present invention utilizes an organic solvent for dissolving the cellulose, so that the blending of cellulose with the synthetic polymer can be facilitated. The mixture thus blended does not include any metallic ions which deleteriously affect the dielectric loss tangent.

DETAILED DESCRIPTION OF THE INVENTION

Prior to describing the invention in more detail, four methods for dissolving cellulose will be briefly indicated as follows:

1. A method for dissolving cellulose employing an organic solvent, an amine, and sulfurous anhydride (SO.sub.2) is described in Japanese Pat. Pub. Nos. 32671/1969, 2113/1970, and Japanese Application No. 38808/1968.

As the amine employed in this method, an aliphatic primary, secondary, or tertiary amine, or an alicyclic secondary amine, for example, isobutylamine, secondary butylamine, tertiary butylamine, dimethylamine, diethylamine, trimethylamine, triethylamine, piperidine, or pyrolydone is advantageously employed. As an organic solvent, following substances can be employed:

Formamide, N-methylformamide, N,N-dimethylformamide, acetamide, dimethylsulfoxide, diethylsulfoxide, acetonitrile, propionitrile, n-butyronitrile, benzonitrile, nitrobenzene, methylene chloride, chloroform, 1,1-dichloroethane, ethylenechloride, .gamma.-butyrolactone, methylthiocyanate, ethylthiocyanate, ethylene carbonate, and propylene carbonate.

Cellulose is added to any one of the above-described solvents so that the cellulose is dispersed in a slurrylike manner in the solvent, and more than 3 mols of an amine and sulfurous acid anhydride respectively for each glucose residue are added so that the cellulose is dissolved.

2. A method for dissolving cellulose employing an organic solvent and nitrogen dioxide is described in Japanese Pat. Publication No. 2114/1970 and Japanese application No. 38809/1968:

Organic solvents not including an active hydrogen, such as N,N-dimethylformamide, N,N-diethylformamide, acetonitrile, propionitrile, diethylsulfoxide, N-methyl-2-pyrrolidone, methyl formate, ethyl formate, methyl acetate, ethyl acetate, n-butyl acetate, methyl propionate, cellosolve acetate, .delta.-butyroloctone, nitromethane, nitroethane, nitrobenzene, 1,4-dioxane, tetrahydrofuran, pyridine, and the like can be employed, and any one of the above-described organic solvents is added to the cellulose so that the cellulose is thereby dispersed. More than 3 mols for each residue of glucose, of nitrogen dioxide as a liquid or a gas are added to the above-described solution so that the cellulose is thereby dissolved.

3. A method for dissolving cellulose employing an organic solvent and nitrosyl chloride (NOC1) is described in Japanese Pat. Application No. 92225/1968:

Cellulose is dispersed into an organic solvent such as dimethylsulfoxide, diethylsulfoxide, N,N-dimethylformamide, N,N-dinethylacetamide, N-methyl-2-pyrrolidone, pyridine and the like, Nitrosyl chloride, more than 3 mols for each glucose residue, is added so that the cellulose is dissolved in the solvent.

4. A method for dissolving cellulose employing an organic solvent and chloral anhydride is described in Japanese Pat. Application NO. 92224/1968:

Cellulose is dispersed into an organic solvent such as dimethylsulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, pyridine, N-methyl-2-pyrrolidone, and the like, and adding chloral anhydride of more than 5 mols per each glucose residue is added so that the cellulose is dissolved in the solvent.

All of the above-described methods for dissolving cellulose can be carried out generally at room temperature and atmospheric pressure, and the time period required for the dissolution ranges from several minutes to several hours. Cellulose to be dissolved in the solvent can be selected from a group consisting of chemical woodpulp, cotton linter, depolymerized cellulose, and regenerated cellulose. However, from the point of view of employing in an electric insulting paper, any kind of cellulose not including any impurity, for instance, nonbleached kraft-pulp, can be employed.

Furthermore, still another method for dissolving cellulose employing substances including no hydrogen is as follows:

5. A method for dissolving cellulose employing liquid sulfurous acid and an amine:

In this method, no organic solvent is employed, and an amine as described in method (1) above is added into the liquid sulfurous acid whereby the cellulose is dissolved at a room temperature and at atmospheric pressure.

Production processes of electric insulating paper according to the present invention will not be described in greater detail.

A cellulose solution is produced firstly employing any one of the five methods described above, and, in this solution, one, two, or more types of synthetic polymers having low dielectric loss tangent are added as a liquid or as a fine powder, thus blending the cellulose with the synthetic polymer or polymers. The blended solution is thereafter extruded into air or into a precipitant for the blended solution so that a regenerative solidification thereof can be obtained. For the purpose of obtaining a pulplike substance or granules thereof to be made into paper from the mixed solidification of the cellulose and the synthetic polymer or polymers, the following methods are suitable.

1. As in the case of spinning viscose or synthetic polymeric threads, the blended solution of cellulose and the synthetic polymer or polymers is spun into air or a liquid, employing a dry or wet method, and, after the spun solution is solidified, the solid material is then cut into short fibers. When these short fibers are beaten, if it is required, and then fibrillated, the pulplike substance to be employed for producing insulating paper can be obtained.

2. Employing a nozzle diameter larger than in the case of item (1) but less than 3 mm., the blended solution is spun into air or into a precipitant from a predetermined height in a manner not exerting any elongating force, so that the spun substance is solidified into fibers. The fibers thus obtained are subsequently cut into a predetermined length and beaten, if it is required, so that the material is thereby fibrillated, and the pulplike substance to be used in insulating paper can be obtained.

3. A blended solution of cellulose and a synthetic polymer is extruded through a nozzle into a precipitant for the blended solution, and the regenerated substance thus obtained is fibrilized into a pulplike substance by rotating quickly the precipitant and employing a shearing force.

4. After the above-described blended solution is solidified regeneratively into a filmlike configuration, the solidified substance is split into a split-fiber, which is thereafter made into pulplike substance through a beating process, if it is required.

5. After the above-described blended solution is solidified regeneratively into any desired configuration, the solidified substance is formed into any granular configuration such as globular, flat, or any other shape.

6. In any of the above-described five methods from (1) to (5), a water-soluble polymer, for instance, methyl cellulose, or ethyl cellulose is further added for the purpose of promoting fibrillation, and after solidification, the water-soluble polymer can be removed by dissolving it during the beating process for fibrillating the solidified substance.

Although the pulplike substance obtained from the blended solution of cellulose and a polymer in accordance with the above-described methods is in itself easily made into paper, if it is required, an ordinary beaten pulp can be further added to the above-described pulp-like or granular substance which is thereafter made into paper, so that the content of the synthetic polymer is about 20 to 80 weight percent of the paper, and, in this way, an electric insulating paper of superior electrical and mechanical characteristics can be obtained.

As the mixing methods of the cellulose and the synthetic polymer, not only merely adding the synthetic polymer into the cellulose solution as described above, but also a method which comprises dispersing cellulose in slurry state into a synthetic polymer solution or a suspension thereof can be employed, and the synthetic polymer can be mixed with the cellulose. As for the synthetic polymer adapted to produce an insulating paper having a low dielectric loss tangent, those having a low dielectric loss tangent and a high heat resistance are desirable. Such polymers can be selected from a group consisting of polycarbonates, polyphenyleneoxides, polysulfones, polystyrenes, cross-linked polyethylenes, polytetrafluorethylenes, polytrifluoroethylene chlorides, polypropylenes, polyacetals, poly-4-methylpentene-1, polyvinylcarbazoles, polyesters, and polyfluoroethylene-propylene copolymers.

Although methods for producing electric insulating papers from a mixed solution consisting of cellulose and a synthetic polymer have been described above, the electric insulating paper of an advantageous quality can also be produced from a solution of graft cellulose.

More specifically, within the above-described solvent systems described in (1) through (4) for cellulose, a solvent system employing an organic solvent soluble for grafted polymers is selected, which system is utilized with another solvent system (5) for cellulose for the purpose of dissolving graft-cellulose, and whereby a graft-cellulose solution can be obtained. According to this method, styrenegraftcellulose, various kinds of vinyl monomer graft-celluloses, and silicone graft-cellulose can be dissolved, and from which solution, a pulplike or granular substance easily made into paper can be obtained. In this way, an electric insulating paper can also be produced from graft-cellulose which has been heretofore difficult to be processed with beating and which has been difficult to be made into paper. When it is desired, one or more types of above-described synthetic polymers can be added to the graft-cellulose solution so that a mixed solution or suspension thereof is obtained, and from this solution or suspension the insulating paper can also be produced.

Kinds of monomers to be grafted to cellulose and the kinds of organic solvents adapted for use and selected from the above-described systems (1) through (4) are indicated in the table shown below. ##SPC1##

The present invention will now be described with reference to specific examples although the invention is not necessarily limited by these examples. In the examples, an insulating paper of more than 100-micron thickness is used from the practical point of view. However, it will be further apparent that insulating papers of a thickness equal or less than 50 microns exhibits better characteristics than those for the more than 100-micron thickness. In the following examples, the measurements of the dielectric constants and dielectric loss tangents have been carried out under the conditions of a 10 kv./mm. electric field strength, 60 cps. frequency, and the insulating paper is immersed with JIS class 1 oil, and impulse breakdown strength is measured at room temperature impregnated with JIS Class 1 oil (sheet test) also. The measured densities are the apparent values obtained at 20.degree. C. and 65 percent R.H., and the gas air-tightness was measured by means of "Oken Type" airtightness measuring instrument. It should be noted that the conventional electric insulating paper which is made of pulp mixed with substances of better insulating characteristics has a structural construction in which the substances of better insulating characteristics are sandwiched between the pulp fibers, whereas the insulating paper according to the present invention has a construction in which the substance or substances of better insulating characteristics are involved in or enveloped by the cellulose fiber itself.

EXAMPLE 1

Three parts of nonbleached kraft pulp for producing insulating paper are added into a mixed solution consisting of 50 parts of N,N-dimethylformamide and 50 parts of dioxane, and the mixed solution thus obtained has 6 parts of nitrogen dioxide added to it and stirred at room temperature under atmospheric pressure. After about 30 minutes of stirring, a cellulose solution of transparent, green-blue, and viscous nature is obtained. A solution consisting of three parts of a polysulfone, P-1700 Union Carbide C., and 15 parts of dioxane is further added to the cellulose solution and stirred, so that a transparent uniformly blended solution of cellulose and polysulfone is obtained. The blended solution is thereafter extruded through nozzles of 0.6 mm. diameter into air under a pressure of 2 kg./ cm.sup.2 and immediately thereafter the extruded solution is placed into water to be solidified, so that a blended substance between the cellulose and polysulfone having a fibrous shape can be obtained. The fibrous substance is cut into lengths of 3 mm. or less, washed with hot water for removing any remaining organic solvent, and subjected to a beating process using a beating machine, whereby a pulplike substance fibrilized as in the case of an ordinary pulp and beat-processed to about 50.degree. SR can be obtained. Eighty parts of the pulplike substance thus obtained is mixed with 20 parts of a nonbleached kraft pulp of 88.degree. SR beating degree to be produced into an insulating paper, and the resultant mixture is made into paper using pure water. The characteristics of the thus produced insulating paper are indicated in table I together with those for a conventional insulating paper made of nonbleached kraft pulp for comparison. --------------------------------------------------------------------------- TABLE I

Insulating Conventional Paper of Insulating this Invention Paper __________________________________________________________________________ Density g./cm..sup.3) 0.56 0.72 Thickness (micron) 130 130 Polymer Content (weight %) 40 Dielectric Constant at 30.degree. C. 2.70 3.57 at 80.degree. C. 2.73 3.59 at 100.degree. C. 2.75 3.61 Dielectric Loss Tangent (%) at 30.degree. C. 0.075 0.210 at 80.degree. C. 0.079 0.212 at 100.degree. C. 0.099 0.246 Impulse Breakdown Strength (kv./mm.) 178.2 125.4 Tensile Strength (kg./mm..sup.2) 6.1 7.3 __________________________________________________________________________

As is apparent from table I, the insulating paper according to the present invention has a dielectric constant, a dielectric loss tangent, and an impulse breakdown characteristic which are far superior to those of the conventional insulating paper made of the nonbleached kraft pulp only.

EXAMPLE 2

Thirty (30) parts of dimethylsulfoxide and 6 parts of diethylamine are added to 3 parts of nonbleached kraft pulp employed for producing insulating paper so that the kraft pulp is well mixed and swollen by these substances. The mixture is then blended with 5 parts of sulfurous acid anhydride in a liquid state and stirred for about 30 minutes at a room temperature. A cellulose of viscous, transparent, and yellowish-brown can be obtained. On the other hand, 3 parts of polyphenylenoxide, PPO 691-111 made by the General Electric Co., is dissolved in 70 parts of chloroform, and the thus obtained solution is further mixed with three parts of diethylamine and one part of sulfurous acid anhydride. The resultant polyphenylenoxide solution is thereafter added to the above prepared cellulose solution little by little and stirred thoroughly so that a blended solution of cellulose and polyphenylenoxide can be obtained. The blended solution is extruded through nozzles of 0.6 mm. diameter into air as described in example I and solidified in a regenerative bath consisting of water and methanol mixed at a ratio of 3:7 so that a fibrous substance is obtained. The fibrous substance is then cut into lengths less than 3 mm., washed with pure water, and beaten to obtain a fibrillated pulplike substance of 63.degree. SR beating degree. Eighty parts of this pulplike substance is mixed with 20 parts of nonbleached kraft pulp of beating degree 88.degree. SR, and then made into paper employing pure water. The characteristics of the thus obtained insulating paper are shown in table II. For the purpose of comparison, the characteristics of a paper made of polyphenylenoxide of granular form less than 150 mesh and of nonbleached kraft pulp for insulating paper are also shown in table II. --------------------------------------------------------------------------- TABLE II

Insulating Paper Polyphenylen- According to This oxide Powder Invention Mixed Paper __________________________________________________________________________ Density (g./cm..sup.3) 0.58 0.66 Thickness (micron) 120 120 Polymer Content (%) 40 40 Dielectric Constant at 30.degree. C. 2.64 2.83 Dielectric Loss Tangent (%) at 30 .degree. C. 0.077 0.089 at 80.degree. C. 0.080 0.094 at 100.degree. C. 0.108 0.125 Impulse Breakdown Strength (kv./mm.) 171.4 93.6 Tensile Strength (kg./mm..sup.2) 5.8 2.0 __________________________________________________________________________

As is apparent from the table II, the insulating paper according to this invention has a superior impulse breakdown strength and tensile strength in comparison with the paper wherein the granular polyphenylenoxide is simply mixed with the kraft pulp.

EXAMPLE 3

Three parts of nonbleached kraft pulp for producing insulating paper is dispersed into a mixture consisting of 30 parts of formamide, 70 parts of chloroform, and 10 parts of diethylamine, and thereafter bubbling into the solution eight parts of sulfurous acid anhydride in gas state, a cellulose solution of viscous, transparent, and yellowish-brown can be obtained. On the other hand, three parts of polysulfone is dissolved into 15 parts of chloroform, and the thus-obtained liquid is added to the above described cellulose solution so that a uniformly blended solution of cellulose and polysulfone is thereby obtained. The blended solution is extruded through nozzles of 06. mm. diameter into methanol rotated at a high speed and cut into short fibers by a propeller with blades. The short fibers are then beaten to 45.degree.0 SR, so that pulplike substance fibrillated can be obtained, 60 parts of the pulplike substance is mixed with nonbleached pulp for producing insulating paper of 75.degree. SR beating degree, and the mixture is made into paper employing pure water. The insulating paper thus obtained has the characteristics shown in table III. --------------------------------------------------------------------------- TABLE III

Insulating Paper of This Invention __________________________________________________________________________ Density g./cm..sup.3) 0.60 Thickness (micron) 135 Polymer Content (%) 30 Dielectric Constant at 30.degree. C. 2.80 Dielectric Loss Tangent (%) at 30.degree. C. 0.098 at 30.degree. C. 0.102 at 100.degree. C. 0.131 Impulse Breakdown Strength (kv./mm.) 165.3 __________________________________________________________________________

EXAMPLE 4

In the example 3, although methanol is employed for solidifying the mixed solution, the solidifying speed of the mixture consisting of cellulose and polysulfone is found to be too fast, whereby the obtained fibers turned out to be too hard and too long of a period was required for beating. To overcome this difficulty, the blended solution of cellulose and polysulfone obtained as in example 3 is extruded through nozzles of 0.6 mm. diameter into water in a stationary state for solidification. Thus solidified substance is thereafter placed into methanol for removing the remaining chloroform completely so that a fibrous substance is regenerated. The regenerated substance is then beaten. Since the substance now obtained can be more easily beaten, the beating degree can be raised to 70.degree. SR. When the pulplike substance thus obtained is made into paper employing the same preparation ratio as in example 3, an insulating paper of higher airtightness and impulse breakdown strength can be obtained. The test results are indicated in the following table IV. --------------------------------------------------------------------------- TABLE IV

Example 3 Example 4 __________________________________________________________________________ Airtightness (sec./100 cc.) 3,000 26,000 Impulse Breakdown Strength (kv./mm.) 165.3 185.0 __________________________________________________________________________

EXAMPLE 5 A blended solution of cellulose and polysulfone prepared according to example 1 was extruded through a spinning nozzle having 30 holes of 70-micron diameter into water, and after the solution was solidified, the solidified fibers were elongated by about 250 percent and blended fibers of cellulose and polysufone were obtained. These fibers were cut to lengths less than 3 mm., beaten to about 70.degree. SR of pulplike substance, and made into paper employing pure water. The insulating paper thus produced exhibited the characteristics shown in table 5. --------------------------------------------------------------------------- TABLE V

Insulating Paper of This Invention __________________________________________________________________________ Density (g./cm..sup.3) 0.60 Thickness (.mu.) 130 Polymer Content (%) 50 Dielectric Constant at 30.degree. C. 2.82 Dielectric Loss Tangent (%) at 30.degree. C. 0.061 at 80.degree. C. 0.064 at 100.degree. C. 0.093 Impulse Breakdown Strength (kv./mm.) 174.5 __________________________________________________________________________

EXAMPLE 6

Twelve parts of polyphenylene oxide, 20 parts of dimethylsulfoxide, 80 parts of methylene chloride, and eight parts of diethylamine were added to three parts of unbleached kraft pulp to be produced into an insulating paper, the mixture was further added to five parts of sulfurous acid anhydride and cooled for about 3 hours at 0.degree. C. In this way, the cellulose was dissolved and a solution of viscous and yellow, with a mixing ratio of cellulose and polymer being about 1:4, could be obtained. This solution was extruded through nozzles of 0.6 mm. diameter into air maintained at 40.degree. C. so that most of the methylene chloride was evaporated. The remaining substance was dropped into methanol for completely removing the remaining dimethylsulfoxide, diethylamine, and methylene chloride, a fibrous substance could be obtained. The fibrous substance was then cut and beaten to 55.degree. SR, and the resultant pulplike substance, 50 parts, was mixed with 50 parts of unbleached kraft pulp for insulating paper and of 88.degree. SR beating degree and made into paper employing pure water. The thus-obtained insulating paper exhibited the characteristics shown in table 6. --------------------------------------------------------------------------- TABLE VI

Insulating Paper of This Invention __________________________________________________________________________ Density (g./cm..sup.3) 0.55 Thickness (micron) 110 Polymer Content (%) 40 Dielectric Constant 2.64 Dielectric Loss Tangent (%) at 30.degree. C. 0.080 at 80.degree. C. 0.084 at 100.degree. C. 0.100 Impulse Breakdown Strength kv./mm.) 170.6 __________________________________________________________________________

EXAMPLE 7

Three parts of cotton linter was dispersed into 100 parts of dimethylsulfoxide, 12 parts of chloral anhydride was added thereto so that the cellulose was dissolved at room temperature into a cellulose solution colorless and transparent. This solution was further mixed with 12 parts of a denatured polyphenyloxide ground to less than 200 mesh, Noryl 731-802, made by the General Electric Co., so that a suspension of a cellulose solution was obtained. The suspension was therefore extruded through nozzles of 1.3 mm. diameter into water in a manner as described in example 1 for solidifying the cellulose. The solidified substance was then beaten by means of a Lumpen Mill so that a granular substance of from 50 to 200 mesh was obtained. Forty parts of this granular cellulose-polymer blended substance was then mixed with 60 parts of unbleached kraft pulp of insulating paper use and beaten to 75.degree. SR, and made into paper employing pure water. The characteristics of the insulating paper thus obtained were as indicated in table 7. --------------------------------------------------------------------------- TABLE VII

Insulating Paper of This invention __________________________________________________________________________ Density (g./cm..sup.3) 0.40 Thickness (micron) 120 Polymer Content (%) 32 Dielectric Constant at 30.degree. C. 2.58 Dielectric Loss Tangent (%) at 30.degree. C. 0.078 at 80.degree. C. 0.080 at 100.degree. C. 0.110 Impulse Breakdown Strength (kv./mm.) 136.4 __________________________________________________________________________

EXAMPLE 8

Six parts of polycarbonate was dissolved into a mixed solution consisting of 60 parts of dioxane and 40 parts of N,N-dimethylformamide, and the mixture was further added to four parts of insulating paper use unbleached kraft pulp and six parts of nitrogen dioxide so that the cellulose was thereby dissolved. The mixed solution of the green-blue color of cellulose and polycarbonate thus obtained was then filtered and solidified by extruding through nozzles having 30 holes of 70-micron diameter into water, and further elongated by 250 percent so that blended fibers of cellulose and polycarbonate were obtained. The fibers were then cut into lengths less than 3 mm. and further beaten for fibrillation to obtain a pulplike substance of about 50.degree. SR. The pulplike substance was then made into paper employing pure water, pressed, and dried, and thereafter dipped into methylene chloride solution for about 10 seconds for dissolving one part of the polycarbonate and for strengthening the bonding force between fibers and also for raising air resistance thereof. The characteristics of the insulating paper thus obtained are indicated in table 8. --------------------------------------------------------------------------- TABLE VIII

Insulating Paper of This invention __________________________________________________________________________ Density (g./cm..sup.3) 0.50 Thickness (micron) 120 Polymer Content (%) 60 Dielectric Constant at 30.degree. C. 2.68 Dielectric Loss Tangent (%) at 30.degree. C. 0.080 at 80.degree. C. 0.082 at 100.degree. C. 0.108 Impulse Breakdown Strength (kv./mm.) 158.1 __________________________________________________________________________

EXAMPLE 9

One hundred parts of formamide and 6 parts of diethylamine were added to three parts of insulating paper use unbleached kraft pulp so that the pulp was thereby wetted and swollen. The thus-swollen pulp was further mixed with five parts of sulfurous acid anhydride in a liquid state so that the cellulose was dissolved. Three parts of electron beam irradiated cross-linked polyethylene was then added to this solution and stirred well to obtain a suspension liquid thereof. The suspension thus obtained was then extruded through nozzles of 2.0 mm. diameter into water for solidifying the cellulose. The cellulose thus solidified was then cut into lengths less than 3 mm., ground by a Lumpen Mill into a granular substance of from 50 to 200 mesh. Forty parts of the granular mixture of cellulose and cross-linked polyethylene was then mixed with 60 parts of insulating paper use unbleached kraft pulp beaten to 88.degree. SR and made into paper having characteristics shown in table 9. From this result, it was made apparent that any polymer lighter than water could be easily mixed with pulp and made into paper employing the method described above. --------------------------------------------------------------------------- TABLE IX

Insulating Paper of This Invention __________________________________________________________________________ Density (g./cm..sup.3) 0.40 Thickness (micron) 150 Polymer Content (%) 20 Dielectric Constant at 30.degree. C. 2.71 Dielectric Loss Tangent (%) at 30.degree. C. 0.067 at 80.degree. C. 0.071 at 100.degree. C. 0.095 Impulse Breakdown Strength (kv./mm.) 130.4 __________________________________________________________________________

EXAMPLE 10

Six parts of polystyrene was dissolved into 100 parts of N,N-dimethylformamide, and the thus-obtained solution was further added with three parts of bleached kraft pulp and five parts of nitrosyl chloride so that the cellulose was thereby dissolved for rendering a blended solution of cellulose and polystyrene. The blended solution was then extruded through nozzles of 0.6 mm. diameter into water wherein a propeller having blades was rotated at a high speed so that the blended solution was solidified into short fibers.

The short fibers were thereafter beaten to about 50.degree. SR of a pulplike substance. Sixty parts of the pulplike substance was mixed with 40 parts of insulating paper use unbleached kraft pulp of 75.degree. SR beating degree and was made into paper employing pure water. The insulating paper thus obtained exhibited characteristics as indicated in table 10. --------------------------------------------------------------------------- TABLE X

Insulating Paper of This Invention __________________________________________________________________________ Density (g./cm..sup.3) 0.60 Thickness (micron) 100 Polymer content (%) 40 Dielectric Constant at 30.degree. C. 2.75 Dielectric loss Tangent (%) at 30.degree. C. 0.84 at 80.degree. C. 0.090 at 100.degree. C. 0.115 Impulse Breakdown Strength (kv./mm.) 154.2 __________________________________________________________________________

EXAMPLE 11

Twenty parts of cotton linter, 40 parts of styrene monomer, and 1000 parts of water containing ceric ammonium nitrate at a concentration of 2.7 x 10 .sup.-3 mol//liter were mixed, and the styrene was grafted by heating the mixture at 70.degree. C. under a nitrogen gas flow. The reaction was continued for about 5 hours so that styrene graft-cellulose of 104 percent degree of grafting was obtained. Five parts of this graft-cellulose was dispersed into 100 parts of ethylacetate, 10 parts of nitrogen dioxide was further added thereto for dissolving the graft-cellulose, and the mixture was extruded through a nozzle having 30 holes of 70-micron diameter into methanol so that the mixture was solidified. The solidified substance was thereafter elongated to 200 percent length for obtaining fibers of styrene graft-cellulose, and the fibers were then cut, beaten to about 45.degree. SR, and the thus-obtained pulplike substance, 80 parts, was mixed with 20 parts of insulating paper use unbleached kraft pulp of 75.degree. SR beating degree to be made into paper employing pure water. The insulating paper, having the characteristics as shown in table 11, was obtained. --------------------------------------------------------------------------- TABLE XI

Insulating Paper of This Invention __________________________________________________________________________ Density (g./cm..sup.3) 0.53 Thickness (micron) 130 Polymer Content (%) 40 Dielectric Constant at 30.degree. C. 2.67 Dielectric Loss Tangent at 30.degree. C. 0.081 at 80.degree. C. 0.087 at 100.degree. C. 0.105 Impulse Breakdown Strength (kv./mm.) 144.3 __________________________________________________________________________

EXAMPLE 12

In a manner similar to example 11, styrene was grafted to cotton linter so that styrene grafted cellulose of a graft ratio of 40 percent was thereby obtained. Five parts of this grafted cellulose and three parts of polysulfone were added to 100 parts of dioxane and dispersed well. The mixture thus obtained was further mixed with nitrogen dioxide so that the grafted cellulose was thereby dissolved and a blended solution of a styrene-grafted cellulose and polysulfone was simultaneously obtained. This blended solution was extruded through a nozzle having holes of 0.6 mm. diameter into water for solidification, and the solidified substance was then cut, beaten to about 65.degree. SR, so that the thus-obtained pulplike substance was thereafter made into paper employing pure water. The insulating paper thus obtained has the characteristics shown in table 12. --------------------------------------------------------------------------- TABLE XII

Insulating Paper of This invention __________________________________________________________________________ Density (g./cm..sup.3) 0.60 Thickness (micron) 120 Polymer Content (%) 44 Dielectric Constant at 30.degree. C. 2.84 Dielectric Loss Tangent at 30.degree. C. 0.071 at 80.degree. C. 0.080 at 100.degree. C. 0.102 Impulse Breakdown Strength (kv./mm.) 168.5 __________________________________________________________________________

EXAMPLE 13

Sulfurous acid anhydride as a gas was cooled at -30.degree. C. so that sulfurous acid in a liquid state could be collected in a pressure proof container, and three parts of polysulfone was added to 100 parts of thus-obtained liquefied sulfurous acid so that the polysulfone was thereby dissolved. Three parts of cotton linter and six parts of diethylamine were then added to the above-described liquefied solution, and by raising the temperature to a room temperature, the cellulose was dissolved. The thus-obtained mixed solution of cellulose and polysulfone was extruded through nozzles of 0.6 mm. diameter into methanol so that the mixture was solidified. After washing the solidified substance was beaten to about 45.degree. SR, and 40 parts of the pulplike substance thus obtained was mixed with 60 parts of insulating paper use unbleached draft pulp of beating degree 88.degree. and made into paper employing pure water. The characteristics of the insulating paper thus obtained are shown in table 13. --------------------------------------------------------------------------- TABLE XIII

Insulating Paper of This Invention __________________________________________________________________________ Density (g./cm..sup.3) 0.50 Thickness (micron) 105 Polymer Content (%) 20 Dielectric Constant at 30.degree. C. 2.85 Dielectric Loss Tangent at 30.degree. C. 0.102 at 80.degree. C. 0.106 at 100.degree. C. 0.134 Impulse Breakdown Strength (kv./mm.) 164.4 __________________________________________________________________________

Claims

What is claimed is:

1. A method of producing an electrical insulating paper comprising the steps of (1) preparing a synthetic polymer cellulose liquid mixture by mixing a synthetic polymer with a member selected from the group consisting of a cellulose and a grafted cellulose solution; (2) extruding the thus-prepared liquid mixture into a medium for regenerating the polymer cellulose as a solid; (3) forming the regenerated polymer cellulose into a configuration adapted to be processed into a paper; and (4) processing the thus formed polymer cellulose into a paper of sheet form; said paper in a sheet form containing a synthetic polymer in a range of from 20 to 80 weight percent.

2. The process of claim 1 wherein said liquid mixture of step (1) is a solution.

3. The process of claim 1 wherein said liquid mixture of step (1) is a dispersion.

4. The process of claim 1 wherein said medium of step (2) is air.

5. The process of claim 1 wherein said medium of step (2) is a liquid precipitant.

6. The process of claim 1 wherein said configuration of step (3) is pulp.

7. The process of claim 1 wherein said polymer cellulose of step (4) is blended with additional beaten pulp containing no synthetic polymer.

8. The process of claim 7 wherein said configuration of step (3) is a granular configuration.

9. The method of claim 1 wherein the synthetic polymer cellulose liquid mixture is prepared using a solvent system selected from the group consisting of (a) a system consisting of an organic solvent, sulfurous acid anhydride, and an amine, selected from the group consisting of aliphatic primary amines, aliphatic secondary amines, aliphatic tertiary amines and alicyclic secondary amines, (b) a system consisting of an organic solvent, and nitrogen dioxide, (c) a system consisting of an organic solvent and nitrosyl chloride, (d) a system consisting of an organic solvent, and chloral anhydride, and (e) a system consisting of liquefied sulfurous acid and an amine selected from the group consisting of aliphatic primary amines, aliphatic secondary amines, aliphatic tertiary amines and alicyclic secondary amines.

10. The method of claim 1 wherein said synthetic polymer is selected from a group consisting of a polycarbonate, a polyphenylenoxide, a polysulfone, a polystyrene, a cross-linked polyethylene, a polytetrafluoroethylene, a polytrifluorochloroethylene, a polypropylene, a polyacetal, a poly-4-methyl pentane-1, a polyvinylcarbazole, a polyester, and a polyfluoroethylenepropylene copolymer.

11. The method of claim 1 wherein said graft cellulose is selected from the group consisting of a styrene grafted cellulose, a silicone-grafted cellulose, and a vinyl-grafted cellulose.

12. An electric insulating paper having superior electrical characteristics, produced by the steps (1) preparing a synthetic polymer cellulose liquid mixture by mixing a synthetic polymer with a member selected from the group consisting of a cellulose solution and a grafted cellulose solution; (2) extruding the thus prepared liquid mixture into a medium for regenerating the polymer cellulose as a solid; (3) forming the regenerated polymer cellulose into a configuration adapted to be processed into a paper; and (4) processing the thus formed polymer cellulose into a paper of sheet form; said paper in a sheet form containing a synthetic polymer in a range of from 20 to 80 weight percent.

13. The electric insulating paper of claim 12 wherein said synthetic polymer is selected from the group consisting of a polycarbonate, a polyphenyleneoxide, a polysulfone, a polystyrene, a cross-linked polyethylene, a polytetrafluoroethylene, a polytrifluorochloroethylene, a polypropylene, a polyacetal, a poly-4-methylpentane-1, a polyvinylcarbazole, a polyester, and a polyfluoroethylenepropylene copolymer.

14. The electric insulating paper of claim 12 wherein said grafted cellulose is selected from the group consisting of a styrene-grafted cellulose, silicone-grafted cellulose, and a vinyl-grafted cellulose.