US3701720A

US3701720A - Manufacture of electrically insulating paper

Info

Publication number: US3701720A
Application number: US879187A
Authority: US
Inventors: Hideo Fujita; Masami Ishitohi; Hiroyoshi Suga; Keishiro Ishiwari; Yoshikazu Yotsui; Tadao Yamashita; Masayuki Edamitsu; Shokichi Moriwaki
Original assignee: Dainichi Nippon Cables Ltd
Current assignee: Dainichi Nippon Cables Ltd
Priority date: 1969-11-24
Filing date: 1969-11-24
Publication date: 1972-10-31
Anticipated expiration: 1989-10-31

Abstract

A METHOD FOR PRODUCING ELECTRICALLY INSULATING PAPER HAVING A REDUCED DIELECTRIC DISSIPATI FACTOR COMPRISES CONTINUOUSLY PASSING PAPER THROUGH A LIQUID MEDIUM WHICH HAS A CONDUCTIVITY OF AT LEAST 10 MICRO MHOS/CM. AND WHICH IS FILLED BETWEEN AT LEST ONE PAIR OF ELECTRODES DISPOSED IN FACING RELATIONSHIP WITH EACH OTHER. VOLTAGE IS

APPLIED TO THE ELECTRODES TO THEREBY REMOVE IONIC SUBSTANCES FROM THE PAPER AND THE TREATED PAPER IS DRIED.

Description

HIDEO FUJITA E AL Oc 3 97 3,101,720 I MANUFACTURE OF ELECTRICALLY INSULATING PAPER Fi l-e d Nov'. 24,1969

4 Sheets-Sheet 31,1972 HIDEO FUJITA ET 3' MANUFACTURE OF ELECTRICALLY INSULATING PAPER Filed Nov. '24, 1969 A 4 Sheets-Sh egt 2 Oct. 31, 1972 HIDEO FUJITA ET AL MANUFACTURE OF ELECTRICALLY, INSULATING PAPER 4 Sheets-Sheet 3 Filed Nov. 24

Ogzt. 31, 1972 HIDEO FUJITA ET AL MANUFACTURE OF ELECTRICALLY INSULATING PAPER 4 Sheet s-Shet FiledNov. 24,

United States Patent 3,701,720 MANUFACTURE OF ELECTRICALLY INSULATING PAPER Hideo Fujita, Masami Ishitobi, Hiroyoshi Suga. Keishiro Ishiwari, Yoshikazu Yotsui, Tadao Yamashita, Masayuki Edamitsu, and Shokichi Moriwaki, Amagasakishi, Japan, assignors to Dainichi Nippon Cables Ltd., Amagasaki-shi, Hyogo-ken, Japan Filed Nov. 24, 1969, Ser. No. 879,187

Int. Cl. B01k 1/00 US. or. 204-132 25 Claims ABSTRACT OF THE DISCLOSURE A method for producing electrically insulating paper having a reduced dielectric dissipation factor comprises continuously passing paper through a liquid medium which has a conductivity of at least micro mhos/cm. and which is filled between at least one pair of electrodes disposed in facing relationship with each other. Voltage is applied to the electrodes to thereby remove ionic substances from the paper and the treated paper is dried.

The present invention relates to a method and apparatus for producing electrically insulating paper, particularly electrically insulating paper having low dielectric loss factor.

Electrically insulating paper is extensively used as an insulating material in electric power cables, electric power capacitors, transformers, etc. With the rapid increase of power demand in recent years, the trend has been toward larger transmission power capacity through higher operating voltages in these electric apparatuses. As transmission voltages grow, the dielectric losses become more and more a limiting factor of transmission power capacity. Accordingly, there is a growing need to decrease the dielectric loss factor of the insulating material. Since the dielectric loss factor of insulating material is proportional to the square of the applied voltage, frequency, and dielectric constant and dielectric dissipation factor (tan 6) of the insulating material, the increase in the applied voltage results in corresponding increase in the dielectric loss factor. Therefore, it is required to reduce the dielectric constant and/ or dielectric dissipation factor of the insulating material to decrease the dielectric loss factor as low as possible.

Although various proposals have so far been made to decrease the dielectric dissipation factor of insulating paper, none of these proposals have proved satisfactory. According to one of the methods for producing insulating paper known in the art, for example, wood chips are intensely digested by kraft process and the resultant kraft pulp is sufiiciently washed with a large amount of deionized water to remove ionic substances. The pulp is then beaten in deionized water to desired extent and made into a cable insulating paper by a Fourdrinier paper machine using deionized water. In accordance with this process, however, the ionic substances in the fibers can not be removed sufiiciently and the finished paper obtained is not satisfactorily low in the dissipation factor. In addition, a long period of washing impairs the mechanical strength of the paper. Although it is possible to attain reduction in the dissipation factor of the paper to some extent at the sacrifice of density, the reduction in density results in a considerable loss of its mechanical strength. For instance, when the paper tape of low density is spirally wound on a conductor and then the paper-wound conductor is bent, wrinkles or tearing may take place in the wound paper.

As disclosed in H. Stamm, M. Kahle, Die Verbesserung 3,701,720 Patented Oct. 31, 1972 ice der elektris-chen Eigenschaften von Kabelund Kondensatorpapier, Wissenschaftliche Zeitschrift der Technischen Hochschule Ilmenau, Jg. 9 (1963) Heft 5, 661-666, another method is also known in which paper is stood in a circulation flow of deionized water having a conductivity of 0.4 to 0.7 micro mho/cm. between parallel electrodes to which DC. voltage is applied. The principle of this method consists in utilizing the gradient of ion concentrations based on the difference of ion content between the paper and the deionized water surrounding the paper. In this method the greater the gradient of ion concentration, the more eminent is the ion diffusion achieved, so that a large quantity of deionized water having low conductivity has to be flowed through the system and moreover it takes a long period of more than one hour to remove the remaining ions sufiiciently. In this method, the application of voltage serves merely to accelerate the ion diffusion.

S. Kh. Kitayeva, V. V. Korolev, Study of the Process of Electrodialysis of Cellulose; Bumazhnaya promyshlennost [Paper Industry] [6], 4-7 (1957) discloses that pulp was subjected to the treatment similar to that described above. However, this procedure requires one hour to reduce ash content of 0.3% to 0.12%, hence undesirable for industrial application. Moreover, cellulose fiber so treated in pulp state is liable to be recontaminated by any foreign materials to increase dissipation factor in the subsequent paper making process.

An object of the present invention is to eliminate these disadvantages of above-mentioned methods and to provide a method and apparatus by which electrically insulating paper having satisfactorily reduced value in dissipation factor can be produced in a period of time far shorter than in above-mentioned procedure.

Another object of the present invention is to provide a method and apparatus for continuously producing electrically insulating paper which is low in dissipation factor and excellent in mechanical strength and which can be applied to high voltage and extra high voltage electrical apparatuses.

Another object of the present invention is to provide a method and apparatus for producing electrically insulating paper which has a uniformly reduced value in dissipation factor at any portion.

Still another object of the present invention is to provide a method and apparatus for producing trouble-free and economically the electrically insulating paper described above on commercial scale.

These and other objects and features of the present invention will become apparent from the following detailed description.

b The object of the present invention will be accomplished (1) Continuously passing paper through a liquid medium having a conductivity of at least 10 micro mhos/cm. and filled between at least one pair of facing electrodes applied with voltage to thereby remove ionic substances from the paper by extraction, and

(2) Drying the paper thus treated.

As a result of the researches conducted by the present inventors, it has been found out that when paper to be treated is continuously passed through a liquid medium having a conductivity of 10 micro mhos/cm. or higher which is filled between a pair of electrodes disposed in facing relationship with each other and applied with voltage, the ionic substances in the paper can be extracted into the liquid extraction medium in a very short period of time with high efiiciency to achieve remarkable reduction in the dielectric dissipation factor. In fact it has been ascertained that when paper is treated in accordance with the method of the invention the dissipation factor thereof is markedly reduced within a short period of from several seconds to minutes. Based upon this new finding the present invention has been accomplished.

The principle of the treatment of the present invention to achieve remarkable reduction in the dissipation factor in such a short period of several seconds to 10 minutes has not been clear yet. However, it may presumably be attributable to the fact that the paper is subjected to higher electric stress induced by the presence of llqllld extraction medium having a conductivity of not less than 10 micro rnhos/cm. so that soluble and hardly soluble ionic substances on the surface and in the interior of the paper are readily dissociated into ions and said ions are shortly extracted into the liquid extraction medium and moreover to the fact that the paper introduced continuously into the electric field releases the ions into the liquid extraction medium to maintain its conductivity at a higher level.

In this respect, the present invention differs substantially from the above-mentioned known methods such as H. Stamms which employ the liquid having a lower ion concentration such "as deionized water (0.4 to 0.7 micro mho/ cm. in conductivity) in order to accelerate ion diffusion resulted from the difference in ion concentration.

In accordance with the present invention, it is required that the liquid extraction medium existing between electrodes has a conductivity of at least 10 micro mhos/cm. If the conductivity is lower than 10 micro mhos/cm., only poor extraction efliciency can be. attained and the treatment requires a long period of more than one hour as in H. Stamms method. In this case even if the applied voltage is stepped up, the extraction efiiciency can not be improved, nor can the period for extraction be shortened. The higher the conductivity of the extraction medium existing between electrodes, the greater is the effect of ion extraction. Preferably, the conductivity of the liquid medium may not be more than 300 micro mhos/cm. Although a liquid medium whose conductivity is higher than 300 micro mhos/cm. can be employed in the present invention, the increase in the conductivity tends to increase power consumption and accelerate generation of heat and gases due to the electrolysis of water. Therefore, the conductivity may preferably be in the range of to 300 micro mhos/ cm., and more preferably, in the range of 30 to 250 micro mhos/cm. The liquid extraction medium having a conductivity of not less than 10 micro mhos/cm. may be present over the whole length of the electrodes but it is not necessarily required to provide the medium in such manner, as far as the liquid extraction medium of 10 micro mhos/cm. or higher conductivity is filled between the electrodes over only effective length of the electrode which is represented by the following equation:

Effective length in meter of electrode=V t in which V is a supply speed (tn/sec.) of paper and t is a period .of time (sec) required for decreasing the dissipa-tion factor of the paper to be treated to the desired extent.

At the start of the operation, the conductivity of the liquid medium to be used in the present invention may not necessarily be 10 micro mhos/cm. or higher but a liquid medium having a conductivity lower than 10 micro mhos/ cm. can also be used, because soluble ionic substances are dissolved into the liquid medium from the paper which is continuously fed into the medium to progressively achieve increase of the conductivity of the liquid medium. Accordingly, in case where a liquid medium having an original conductivity of less than 10 micro mhos/ cm. is used, paper is continuously supplied into the liquid medium with or without the application of voltage prior to the extraction operation until the conductivity thereof reaches to 10 micro mhos/cm. or higher, and thereafter the operation of the present invention is initiated. As the liquid extraction medium, water or aqueous solutions containing various chemicals dissolved therein are economical and effective. The present invention is especially advantageous in that it can employ industrial water which is available at low cost. In order to elevate the conductivity of water, at least one of ionic substances which are soluble to dissociate into ions may be added to the water. The amount of the ionic substances to be added, if properly selected, enables the operator to adjust the conductivity of water to an appropriate level. Preferably employable as the ionic substances are water-soluble salts of inorganic or orgamc acid, insofar as they exert no adverse influence on the paper to be treated and on the electrode. The preferable inorganic salts are ammonium, alkali metal and alkaline earth metal salts of inorganic acids such as hydrochloric acid, sulfuric acid, phosphoric acid, carbonic acid, etc. Preferable examples are NH C1, (NH SO NH NO' NaNO NaCl, Na SO Na CO NaHCO KCl, K K CO KNO3: Za 3)29 i: Ba(NO BaCl etc. The preferable salts of organic acid are ammonium, alkali metal and alkaline earth metal salts of various organic acids, such as acetic acid, oxalic acid, etc. Also employable are water-soluble complex salts and double salts of various metals, such as potassium chromium sulfate, etc. Other ionic substances such as acid or alkali may also be added to water in such amount that the addition of the acid or alkali does not produce undesirable eifect on the quality of the paper to be treated and on the electrode. The acids to be added may be organic or inorganic acids, but preferable are inorganic acids, such as hydrochloric acid, sulfuric acid, sulfinic acid, nitric acid, nitrous acid, hydrocyanic acid, carbonic acid, phosphoric acid, phosphorous acid, metaphosphoric acid, boric acid and chloric acid. Examples of the organic acids are carboxylic acids such as acetic acid, oxalic acid, etc. and aminoacids. The alkalis to be added include potassium hydroxide, sodium hydroxide, calcium hydroxide, magnesium hydroxide, barium hydroxide and ammonium hydroxide, the most preferable being calcium hydroxide and magnesium hydroxide. Further, mixtures of water and water-soluble nonionic organic substances may also be used, the representative of these being various Watersoluble organic substances such as water-soluble ones among monohydric alcohol, glycol, aldehyde, ketone, ketone-alcohol, ether, ester and urea. The water-soluble nonionic organic substances described may be added to water together with one or more of ionic substances as above mentioned. The amount of the ionic substances to be added may be determined depending upon the desired conductivity of the liquid, but under any circumstances the addition should not be made in an amount which would affect the quality of the paper adversely and which is detrimental to electrode.

Preferable as paper to be treated in accordance with the present invention is electrically insulating paper produced by conventional methods and known to the skilled in the art. The electrically insulating paper may be classified into a wide variety of types and grades including coil insulating paper, capacitor tissue paper, high voltage condenser paper, cable insulating paper, extra high voltage cable insulating paper and the like. Papers-of all of these types can be treated in accordance with the present invention to reduce the dissipation factor. Of these papers the method of the invention may advantageously be applied to conventional high voltage insulating paper having an apparent density of 0.5 to 1.3 g./cm. and an air resistance at least 150 Gurley sec/ cc. Preferably applicable are the extra high voltage insulating paper having an apparent density of 0.55 to 0.95 g./cm. and an air resistance of 200 to 5000 Gurley sec/100 cc. Inaddition, the invention is applicable to special insulating paper with improved electrical and mechanical characteristics such as mica loaded insulating paper described in T. Yamarnoto, M. Yamamoto, S. Nakamoto, and Y. Take Mica- Loaded Paper for EHV Power Cable transaction paper No. 69 TP 97-PWR, presented at the IEEE Winter Power Meeting, New York, N.Y., Jan. 26-31, 1969, Combination insulating paper described in K. Kojima, H. Kubo and S. Maruyama Development of Special Insulating Paper for EHV Power Cable transaction paper No. '68 TP 64-PWR, presented at the IEEE Winter Power Meeting, New York, N.Y., Jan. 28-Feb. 2, 1968, or insulating paper made on the multiwire principle in which the plies of paper are brought together in the wet state and formed into a single sheet. However, the paper to be treated by the present invention is not limited to the insulating papers mentioned above, but the present method is also useful to paper such as is too poor to use as insulating paper with respect to electrical characteristics, particularly to the dissipation factor. Such paper can also be used for insulating purposes after being treated by the method of the present invention. Furthermore, the present invention is applicable to paper dried after being made into sheets, as well as to undried paper which was not thoroughly dried or which was half-dried or undried. The type of the paper to be treated by the present method may be selected depending upon the purposes for which the insulating paper obtained is used.

The speed of paper to be passed through between the electrodes depends upon the thickness, apparent density and strength of paper, the amount of the ionic substances to be extracted from paper, applied voltage, length of the electrodes, conductivity and flow rate of the liquid extraction medium and other conditions. However, the speed of paper is controlled within such range that the paper is not subjected to excess mechanical tension. It is in the range of 0.1 to 800 m./min., preferably 1 to 500 m./min., and more preferably in the range of to 300 m./min. The period of time for the paper to be subjected to the electric stress between the electrodes (i.e. treating time) depends on the above-mentioned speed of the paper and on the length of the electrodes. Desired results can usually be obtained in several seconds to 10 minutes. his most desirable that paper may be fed in with the paper surface facing the electrode surface since the best extraction efliciency can be attained in this position, but slight inclination relative to the electrode surface, if any, may not be objectionable.

The voltage to be applied to the electrodes is preferably direct current voltage, but it is also possible to use alternating current voltage whose half cycle is equal to or longer than such period that ionic substances in the paper are dissociated into ions and the resulting ions are then extracted into the liquid extraction medium. Applicable is the alternating current voltage having a frequency of less than 1 kHz. As shown in Example 12 described hereinafter, the effect of the present invention can of course be achieved by the application of an alternating current voltage having a commercial frequency. These alternating current voltages may also be superposed on direct current voltage. A high voltage may be applied so far as the voltage does not give rise to electrical failure of the paper. Appropriate voltage to be applied to the electrodes is in the range of l to 800 v. per cm. of the distance between the electrodes, preferably in the range of 15 to 500 v. per cm. of the distance, though it depends also on the conductivity of the liquid extraction medium.

Although the temperature of the liquid medium may be elevated due to the flow of current, it is not desirable that the temperature thereof increase to excess. Preferably, it may not exceed 80 C., most preferably, it may not be higher than 60 C. When desired, therefore, the vessel may be cooled externally.

In accordance with the present invention, the liquid extraction medium itself may be static or mobile, but it is preferable to flow the liquid extraction medium at a constant rate in a definite direction in order to obtain paper having a uniform dissipation factor at any portion of the paper. When the extraction treatment is carried out under the condition that liquid extraction medium is flowed at a constant rate in a definite direction, stationary distribution in conductivity of liquid extraction medium can be achieved. Such flow state of the liquid extraction medium can be easily attained by continuously supplying the medium into one side of the vessel and discharging said medium from the other side of the vessel in such a manner that the liquid level in the vessel is kept constant. The liquid extraction medium may be flowed in the same direction, in the counter direction, in the perpendicular direction or in some other desired direction with respect to the moving direction of paper, but the same or counter direction is preferable to establish the stationary state in conductivity distribution easily. The flow rate of the liquid extraction medium is determined depending upon the feeding rate of paper, thickness and apparent density of paper, the amount of ionic substances in paper, kind of liquid extraction medium and its conductivity, applied voltage, and length of electrodes. It is in the range of 0.05 to 500 m./min., preferably in the range of 0.5 to 200 m./min.

In case where the liquid extraction medium is flowed between the electrodes, hydrogen gas and oxygen gas generated by electrolysis of water are carried away from the space between the electrodes in the form of small bubbles together with the medium flow, removal of the generated gases being thus facilitated. At the same time, the aforementioned elevation of temperature of the liquid extraction medium can also be suppressed. Continuous movement of paper at constant rate and constant flow of the liquid extraction medium ensure uniform extraction treatment.

Although the surface of the paper treated in accordance with the present invention may be recontaminated with ionic substances contained in the liquid medium used, such ionic substancescan readily be removed from the surface of the paper by washing with water having a lower conductivity after extraction treatment. However, in the case where the treatment is performed with a liquid extraction medium having a relatively low conductivity in the order of 10 to 20 micro mhos/cm., the washing process may be omitted, provided the treated paper is not desired to be of extremely low value in dissipation factor. If it is desired to obtain paper whose dissipation factor is reduced to a very low level of about 0.14%, the conductivity of the washing liquid may not be more than 6 micro mhos/cm., preferably not exceeding 2 micro mhos/cm., while for the production of paper whose dissipation factor is approximately 0.20%, the conductivity of the Washing liquid to be used is not more than 18 micro mhos/cm., preferably not more than 10 micro mhos/cm. In general, washing water having a conductivity of not more than 20 micro mhos/cm. may be used.

The features of the present invention will become apparent from the detailed description of preferred embodiments of the apparatus to be used in carrying out the present method, the explanation being given with reference to FIGS. 1 to 4.

FIG. 1 is a side elevation partly in vertical section showing a preferred embodiment of the apparatus for practicing the method of the present invention;

FIG. 2 is a view of an extraction vessel in section taken along the line XX in FIG. 1;

FIG. 3 is a side elevation partly in vertical section showing another preferred embodiment of the extraction vessel; and

FIG. 4 is a side elevation partly in vertical section showing another preferred embodiment of the extractionvessel.

Referring to FIGS. 1 and 2, designated at 1 is an extraction vessel made of a nonconductive material such as a synthetic resin like, for example, rigid polyvinyl chloride resin, polypropylene resin, phenol resin, polytetrafluoroethylene resin. The vessel 1 is provided with a pair of electrodes disposed in facing relation with each other, namely an anode 2 made, for example, of platinum and a cathode 3 made, for example, of mercury. The platinum anode 2 is formed of a thin platinum plate and fixed to an electrode support 4 made of a heat resisting synthetic resin such as tetrafluoroethylene resin, phenol resin, polypropylene resin or the like. To fix the anode 2 to the support 4, an adhesive or screws may be used. By means of bolts 6 the electrode support 4 is supported on a mounting plate 5 made of a synthetic resin such as phenol resin in vertically movable manner. The plate 5 is further secured, at its Opposite sides, to flanges 7 along the edge of the vessel 1 by screws 8. At one end, the support 4 is bent upward and the anode 2 is connected to a lead member 9 which is fixed to the upward portion of the support 4 and further connected to the power source (not shown). The mercury cathode 3 is formed of mercury which is placed in a space defined by the inner wall of the vessel 1 and a partition wall 10 made of a synthetic resin and extending from the bottom of the vessel 1. Designated at 11 is a lead member of the cathode which is connected to the power source (not shown).

The vessel 1 is further provided with an inlet duct 12 for supplying a liquid medium into the vessel 1 and an outlet duct 13 for discharging the liquid medium from the vessel 1. The ducts 12 and '13 are respectively provided with

valves

14 and 15. Indicated at 16 is a convolute roll of paper P to be treated. The paper P is unwound from the roll 16 at a constant speed by a pair of pinch rolls 17 and fed into the vessel 1 to be passed between the

electrodes

2 and 3 immersed in the liquid medium M by being guided over guide rolls 18, 19 and 20. The paper is then taken. out of the vessel 1 and passed over the guide roll 21. Of these guide rolls, the rolls 19 and 20' are supported respectively on

bearings

22 and 23 in the vessel 1 for free rotation. The convolute paper roll 16, free to rotate, is supported on a suitable frame (not shown), and a pair of the pinch rolls 17 is also supported on a frame (not shown) in rotatable manner, one of the pinch rolls 17 being adapted to be driven by drive means (not shown) at a constant speed. Further the guide rolls 18 and 21 are supported on unillustrated support means in idly rotatable fashion.

The paper taken out of the vessel 1 is led along guide rolls 24, 25, and 26 and introduced into a washing reservoir 27 from which it is taken out by means of pinch rolls 29. In this step, the paper surfaces are subjected to washing water ejected from nozzles 28. The guide roll 24 is supported on a frame (not shown) for idle rotation, and the guide rolls 25 and 26 are received on bearings in the reservoir 27 in freely rotatable manner.

A pair of the pinch rolls 29, free to rotate, is mounted on an unillustrated frame and one of the pinch rolls is adapted to be rotated by drive means (not shown) at the same speed as the .pinch roll 17 for unwinding the paper from the convolute roll 16. The nozzles 28 are secured to the ends of washing water ducts (not shown) supported on an appropriate frame (not shown). Designated at 30 are heating rolls which are adapted to be heated with steam introduced into the interior thereof, the respective rolls being mounted on support frames (not shown) in freely rotatable. manner. When the drying means serves also as taking-out means for treated paper the pinch rolls 29 may be omitted. As paper unwinding 'and take-up means, thosecomrnonly used in paper making, printing and coating processes may be employed in the present invention. The drying means above is also known and used in a paper making factory, but it is not limited to the roll but an infrared ray heater may also be employed for drying operation. Numeral 31 indicates a take-up roll supported on a frame (not shown) in rotatable fashion and adapted to be driven by a motor (not shown).

In the case where paper is to be treated in the liquid medium whichis in static state, the liquid medium is supplied through the inletduct 12 until the space between theanode and cathode is filled and the liquid M reaches an appropriate level while the valve 15 is closed. If paper is to be treated in the liquid medium stream which flows in the direction opposite to that of paper feeding, the

liquid medium is supplied at a constant rate through the inlet duct 12 and overflowed from the outlet duct 13 with both of the

valves

14 and 15 kept open to keep the liquid level constant. On the other hand, when the liquid me- I dium is flowed in the same direction as the feeding direction of the paper, the liquid medium is supplied, on the contrary, through the duct 13 and overflowed through the duct 12.

Although the apparatus shown in FIG. 1 is provided with a pair of facing electrodes, several pairs of facing electrodes may be arranged along the desired moving direction of paper. Each pair of these electrodes can be disposed in any direction such as horizontal, vertical or inclined.

The electrode may be in desired shape as in flat or curved plate form. The distance between the electrodes may be so adapted as to permit the liquid medium to flow and the paper to move in desired manner. Insofar as trouble-free movement of the paper is ensured, the, paper may be in contact with the surface of the electrode. With a mercury cathode, however, -a small gap may preferably be provided between the surface of mercury and the paper to leave the mercury surface undisturbed. If the other conditions are the same, too great a gap is not desirable because the electric stress applied to the paper is lowered, preferable distance between the electrodes being up to 5 cm. accordingly.

While various materials may be used for electrode, desirable as an anode material is platinum or carbon, because they are hardly dissolved into ions. As a cathode material, platinum, carbon or mercury is preferable. Aluminium, inon, gold, silver, nickel, lead or the like may also be used for the cathode. Mercury may advantageously be employed as the cathode material because any deposit on the cathode is automatically carried away to the peripheral edge portions of the mercury electrode and the electrode surface therefore remains fresh all the" time. The more preferable combinations of anode material and cathode material are platinum-platinum, platinum-mercury, platinum-carbon, carbon-carbon, carbon-mercury and carbonplatinum. In an instance where the electrodes are arranged horizontally as illustrated in FIG. 1, the upper electrode may have a structure which readily permits the escape of gases (oxygen and hydrogen) generated by electrolysis of the liquid extraction medium. For instance, -a plurality of electrodes may be spaced apart by a suitable distance to allow the gases to escape through the gaps, or a number of umbrella-like electrodes with gas vents at respective top portions may be connected with one another.

After being subjected to extraction, the paper is passed through the vessel containing washing water and if necessary, the paper is further washed with washing water ejected from a nozzle as seen in FIG. 1. As another embodiment of a washing device an elongated troughlike washing vessel may be used. In this washing vessel, paper taken out of the extraction vessel is fed in at one end, washed with a flow of washing water while being sent forward in the trough, and led out of the other end. The

washing water is supplied from the paper outlet sidev and flowed to the paper inlet side and discharged therefrom. Furthermore, the washing device may be incorporated in the extraction vessel as shown in FIG. 3.

The same reference numerals in FIG. 3 as those in FIG. 1 designate the members similar to those in FIG. 1. In addition to the structure described in the foregoing embodiment, the vessel 1 is provided with

guide plates

32 and 33 facing each other, the electrodes .2, 3 and guide

plates

32, 33 being arranged in series. These

guide plates

32, 33 are made of phenol resin and the upper plate 32 is attached by bolts 34 to a mounting plate 35 in vertically movable manner in the same fashion as the anode support 4, the mounting plate 3-5 being fixed to the flange 7 of the vessel 1 by screws 36. On the other hand, the lower guide plate 33 has legs 37 fitted into supports 38 fixed to the bottom of the vessel 1. Positioned between the

electrodes

2, 3 and the

guide plates

32, 33 is the open end of a duct 39 for supplying the liquid extraction medium between the

electrodes

2 and 3. The duct 39 is provided with a valve 40 for adjusting the supply of the liquid medium and projected into the vessel through the bottom thereof, the duct 39 being driven through a bearing nut 41 fixed to the vessel bottom. Supplied into the vessel 1 through a supply duct 42 at a paper outlet side is washing water. The duct 42 is provided with a valve 43 and extends into the vessel 1 through the side wall at the paper taking-out side of the vessel 1, the opening of the duct 42 being disposed close to the paper. The duct 42 is driven through, and supported by, a bearing nut 44 secured to the vessel 1. The washing water W supplied through the duct 42 is flowed mainly between the guide plates 3'2 and 33 along the passage of the paper. Each of the

bearing nuts

41, 44 for the ducts 39, 42 is made of a synthetic resin such as rigid polyvinyl chloride, polypropylene or the like. When desired, a sealing material is inserted between the nut and the duct. Designated at 13 is an outlet duct for the liquid extraction medium mixed with the washing water disposed at the paper inlet side of the vessel 1. The washing water flows from the open end of the duct 42 toward the outlet duct 13. In the apparatus in FIG. 3, the liquid extraction medium between the

electrodes

2 and 3 is a mixture comprising washing water supplied through the duct 42 and a liquid medium supplied through the duct 39. While passing between the

electrodes

2 and 3, paper is subjected to the extraction treatment, the paper thus treated thereafter being washed satisfactorily with the washing water while it is further driven forward between the

guide plates

32 and 33. The provision of the guide plates is useful in controlling the flow of the washing water in desired manner, although the plates may be omitted. Further, the guide plates may serve as electrodes when they are made of an appropriate electrode material and applied with voltage. In this case, the washing water between the guide plates is very low in conductivity and therefore ionic substances in the paper are hardly extracted by the action in accordance with the present invention but the ions stuck to the surface of the paper or those in the washing water in the vicinity of the paper surfaces are removed upon ion diffusion, thus washing being done more efiectively. In the apparatus shown in FIG. 3, the paper, after being subjected to extraction treatment in vessel 1, washed and led out of vessel 1, may further be washed in the washing device as shown in FIG. 1, when required and then dried and wound in the same manner shown in FIG. 1.

FIG. 4 illustrates the extraction vessel 1 which is provided with pairs of electrodes and a plurality of inlet ducts for supplying a liquid medium and/or washing water. In this drawing similar members are indicated at reference numerals similar to those in FIG. 1. The mercury cathode 3 is placed on the bottom of the vessel 1 to an appropriate depth and is connected to the power source (not shown) through the lead member 11. In facing relationship with the mercury cathode 3 are three platinum anodes 2 which are constructed in the same manner as in FIGS. 1 to 3. As in the case of FIG. 1, paper is guided from the left to the right of the vessel 1 by means of

rolls

19 and 20. Disposed at the paper outlet side are inlet ducts 45, 46 and at center portion, two inlet ducts 47, 48, these ducts being provided with openings directed to the paper supply side. Designated at 49 is a liquid discharge duct which extends into the vessel 1 through the bottom thereof and which has an opening disposed at an appropriate level. These ducts are made of a synthetic resin. The ducts 45, 47 are fixed in position by clamps 50, 51 which are secured to the mounting plates 5 respectively supporting the anodes. The

ducts

46, 48, 49 are respectively supported by hearing nuts 52, 53, 54 made of a synthetic resin and secured to the vessel. When necessary, sealing materials are disposed respectively between the bearing nuts 52, 53, 54 and

ducts

46, 48, 49.

For better understanding of the present invention examples will hereinafter be given.

EXAMPLE 1 Thickness (mm.) 0.100 Apparent density (g./cm. 0.67 Air resistance Gurley sec./ cc.) 2,230 Tensile strength (kg/mm?) 6.41 Elongation (percent) 2.52

The insulating paper A was subjected to extraction treatment by using the apparatus shown in FIG. 1. The apparatus used comprised a vessel 28 cm. in length, 28 cm. in width, 12 cm. in depth; mercury serving as a cathode and filled in a container 25 cm. in length, 25 cm. in width, 1 cm. in height; and a platinum plate 20 cm. in length, 10 cm. in width disposed above the mercury cathode and serving as an anode, the platinum anode being spaced apart by 2 cm. from the mercury cathode. 10-cm. wide insulating paper A was fed into the vessel along the guide rolls 18, 19 and passed continuously at the rate of 120 cm./min. (treating time: 10 sec.) between the platinum anode and mercury cathode in facing relationship with the surfaces thereof but out of contact with the electrodes, while deionized water having a conductivity of 1 micro mho/cm. was continuously introduced into the vessel through the inlet duct 12 at the rate shown in Table 1 along the paper surface. The water was overflowed from the outlet duct 13, whereby the water level in the vessel was maintained constant. D.C. voltage given in Table 1 was applied to the electrodes. In a few minutes the conductivity of water just below the paper at the end of the platinum electrode on the paper inlet side and the conductivity of the discharged liquid increased and became stable to reach the same definite value of not less than 10 micro mhos/cm. The conductivity of the discharged water is shown in Table 1 below. The paper passed through between the electrodes in such a stationary state of conductivity distribution was fully washed by being passed through the reservoir 27 filled with washing water and then spraying deionized water having a conductivity of 1 micro mho/cm. at the rate of 50 to 200 ml. per 100 cm. of the surface area of the paper. The paper was then dried after being drained by means of pinch rolls 29 and treated paper was thus obtained.

The dielectric dissipation factors at 100 C. of the resultant paper evaluated in the state of oil-impregnated paper by the following method are shown in Table 1.

Measurement of dielectric dissipation factor of oilimpregnated paper The evaluation of the dielectric dissipation factor of the paper obtained was carried out in accordance with 118 C-2111 (1967), except that the paper specimen was first dried roughly at a temperature of C. for 10 hrs. in the air, secondly the dried paper was completely dried at a temperature of 120 C. for -4 hrs. under a pressure of about 0.1 mm. Hg after being placed between flat electrodes specified in HS C-2111, thirdly the paper was impregnated with alkyl benzenes having branched alkyl chain whose dissipation factor at 80 C. was not more than 0.005% and whose average molecular weight was about 258, and then the impregnated paper was measured at a stress of 10 kv./mm.

In this specifiaction all values of the dissipation factor of paper were determined in the same manner as above, unless otherwise specified.

In this specification the conductivity of liquid medium was measured by employing a measuring electrode which comprises platinum wires (a), (a), 1 mm. in diameter and 20 cm. in length and spaced apart by 1 mm. as shown in FIG. 5. (The electrode constant -of the measuring electrode was measured in advance employing a calibrated reference electrode.) The measuring electrode was placed at the measuring position, connected through lead wires to a Wheatstone bridge "(not shown) provided with an input power source of 5 v. with frequency of 1 kHz., the bridge was balanced, and AC. conductivity between the two 12 0.177% and hardly any appreciable improvement of the dissipation factor was found as compared with 0.180% which was the value prior to extraction.

The treated paper obtained by carrying out the same treatment with distilled water serving as a liquid medium and having a conductivity of 1.0 micro mho/cm. was found to have a dissipation factor of 0.179% at 100 C., and no improvement of the dissipation factor was seen. In this case in 1 to 2 minutes after voltage application, the conductivity reached a maximum value of about 7.0 micro mhos/cm. and lowered to 2.2 micro mhos/cm. in 5 minutes, thereafter showing approximately a constant value. After lapse of 150 minutes, the value was still 3 micro mhos/cm.

platinum wires was measured. 15

TABLE 1 Conductivity Current 100 0. tan 6 of discharged Applied under of treated water in sta- Rate of deionized water supplied voltage stationary paper obtained tionary sate into vessel (liter/min.) (v state (2.) (percent) u/cm.)

Insulating paper A untreated 0. 177-0. 180

For comparison the, following

Comparisons

1 and 2 35 were conducted.

Comparison 1 In this Comparison 1 the same apparatus as in Example 1 was used. Insulating paper A, 9 cm. in width, was secured between the electrodes in facing relationship with the surfaces thereof but out of contact with the electrodes. About 6 liters of distilled water having a conductivity of 0.55 micro mho/ cm. was placed into the vessel and a D.C. voltage of 300 v. was applied to the electrodes for 2.5 hours.

During the application of D.C. voltage, the conductivity of the distilled water was measured (the measuring position was immedately below the paper), and it was found that the conductivity of the liquid was temporarily increased to a maximum value of about 6.0 micro mhos/ cm. in 1 to 2 minutes after the application of the voltage, then lowered to 2.0 micro mhos/cm. in 5 minutes, thereafter showing an approximately constant value. When 150 minutes elapsed, the value was still 2.5 micro mhos/cm. After. 2.5 hours of the application of the voltage the paper thus treated was taken out of the vessel and sufiiciently washed with deionized water having a conductivity of 0.5 micro mho/cm. and dried.

The dissipation factor at 100 C. of the resultant paper measured in the same manner as in Example 1 was Comparison 2 Treated paper was obtained in the same manner as in Comparison 1 except that 9-cm. wide insulating paper A was secured in the vessel with the paper surface facing the electrodes and immersed in deionized water whose conductivity was 1.0 micro mho/cm. In 10 minutes after the immersion of the paper D.C. voltage was applied. Dissipation factor at 100 C. of the paper obtained was found to be 0.174%. The conductivity of the deionized water was measured at a position just below the paper. In 8 minutes after the immersion of the paper it was 5 micro mhos/cm., and 4.4 micro mhos/cm. in 10 minutes. After the application of D.C. voltage, the value of conductivity increased to 7 micro mhos/cm. in 1 minute, then lowered to 2 micro mhos/cm. in 2 minutes and reached 4 micro mhos/ cm. in 30 minutes.

EXAMPLE 2 Treated paper was obtained in the same manner as in Example 1 except that 9-cm. wide insulating paper A wassent forward at a higher speed of 240 cm./min. (treating time, 5 sec.) and the different voltages given in Table 2 were applied and that deionized water having a conductivity of 1 micro mho/cm. was suppliedinto the vessel at the different rates given in Table 2. The dissipation factor at 100 C. of the paper is also given in Table 2.

TABLE 2 Conductivity Current 100 0. tan 6 of discharged D Applied under of treated water in sta- Rate of deionized water supplied voltage stationary paper obtained tionary state into vessel (liter/min.) (v.) state (a.) (percent) (nu/cm.)

Insulating paper A untreated 0. 177-0. 180

In any of the cases listed above, remarkable improvements of the dissipation factor were achieved. It will be noted that under the conditions of low flow rate and high conductivity of the liquid extraction medium, outstanding improvements in the dissipation factor were attained.

EXAMPLE 3 By employing the same apparatus as in Example 1, insulating paper A cm. in width was fed at a velocity of 1200 cm./min. (treating time: 1 sec.) and distilled water having a conductivity of 1 micro mho/cm. was supplied into the vessel at a rate of 0.5 liter/min. in the direction opposite to the paper moving direction with application of a DC. voltage of 500 v. In a few minutes after the application of the voltage the conductivity of the discharged water reached the constant value of 130 micro mhos/cm. The paper passed between the electrodes in such stationary state was washed with deionized water having a conductivity of 0.5 micro mho/cm. and dried.

The paper thus treated was found to have a dissipation factor of 0.142% at 100 C.

A similar treatment performed with a supply of the above-mentioned distilled water at a rate of 1 liter/min. resulted in a dissipation factor of 0.148% at 100 C. In this case the conductivity of the water reached a constant value of 75 micro mhos/cm. after about 6 minutes of the application of the voltage.

EXAMPLE 4 Treated paper was produced in the same manner as in Example 1 except that industrial water having a conductivity of 150 micro mhos/cm. was supplied into the vessel at a rate of 0.1 liter/min. with application of a DC. voltage of 100 v. Dissipation factor of the paper obtained was 0.145% at 100 C. The conductivity of the discharged water in stationary state was 180 micro mhos/ The insulating paper thus obtained was spirally wound on a conductor 51.0 mm. in outer diameter (and 1500 mm. in sectional area) to a thickness of 19.5 mm. and impregnated with dodecyl benzene after drying in vacuum to produce a 275 kv. oil-filled cable. Untreated insulating paper A was also wound around a conductor of the same size to the identical thickness and impregnated with dodecyl benzene after drying in vacuum to obtain a conventional 275 kv. oil-filled cable. Various characterfrom 1550 kv. and a negative impulse voltage was applied to the conductor three times at each increased voltage level to fail out. The values in the list are represented by maximum impulse breakdown strength on the conductor calculated from the breakdown voltage values. Further, to determine maximum A.C.-long-time breakdown stress, the voltage applied to the cable was increased stepwise by a step of 15 kv. each stage from 520 kv. and each increased voltage level was maintained for 3 hours to cause breakdown. The listed values are represented by maximum breakdown strength on the conductor calculated from the breakdown voltage values.

The results given in the table show that the power cable made of the insulating paper produced in accordance with the present method has electrical characteristics much superior to conventional cable. Both cables were dissected and each composite material was inspected. There were no wrinkles or tearing in both of the treated and untreated paper.

EXAMPLE 5 Five kinds of liquid media of a conductivity of 80 micro mhos/cm. at 20 C. shown in Table 4 were prepared by dissolving various chemicals in deionized water or a mixture of equi-volnme deionized water and methyl alcohol. Extraction treatment was carried out in the same manner as in Example 1, except that the liquid media thus prepared were supplied into the vessel respectively at a rate of 0.1 liter/min. and a D-.C. voltage of 300 v. was applied. The dissipation factor at 100 C. of the paper thus treated was set forth in Table 4, which shows that a satisfactory improvement of the dissipation factor was achieved in each case.

istics of these cables are listed in Table 3 for comparison. Of these characteristics, dissipation factor was measured by a Schering bridge; electrostatic capacity, by an ordinary capacitance bridge; insulation resistance, by a direct deflection method employing a galvanometer; and equivalent dielectric constant was calculated from the electrostatic capacity. To determine maximum impulse breakdown stress of the cable, the voltage applied to the cable Treated paper was obtained in the same manner as in Example 1 except that the distance between the electrodes was fixed at 4.0 cm. and the deionized water having a conductivity of 3 micro mhos/cm. was supplied at the rate listed in Table 5 and that the voltage given in Table 5 was applied. Dissipation factor at 100 C. of the paper obtained was measured, the results being shown in was increased stepwise by a step of 10 kv. each stage Table 5.

TABLE 5 100 0. tan a Conductivity Current of treated of discharged Applied under paper water in sta- Rate of deionized water supplied voltage stationary obtained tionary state into vessel (liter/min.) (v.) state (a.) (percent) u/cm.)

Insulating paper A untreated 0. 177-0. 180

EXAMPLE 7 ample 1, except that the insulating paper B cm. in width at the rate shown in Tables 6 and 7 below and insulating paper A 10 cm. in width was fed at rates of 120 cm./min. (treating time: 10 sec.) and 600 cm./min. (treating time: 2 sec.) respectively. The results are presented in Table 6 (treatment at 120 cm./min.) and in Table 7 (at 600 cm./ min.). From the comparison of Table 6 with Table 7, it

is apparent that the higher the paper feeding speed, the

was fed at rates of cm./min. (treating time: sec.) and 3 cm./min. (treating time: 400 sec.) respectively, while deionized water having a conductivity of 1 micro mho/cm. was supplied into the vessel at a rate of 2 liters/ min. with the application of the voltage shown in Table 8. After being washed with deionized water having a conductivity of 10 micro mhos/cm., the paper obtained was dried. The dissipation factor at 100 C. of the treated paper was measured and the results are shown in Table 8. For comparison, Table 8 includes the dissipation factor at 100 C. of untreated insulating paper B and that of the insulating paper which was only washed with deionized water having a conductivity of 10 micro mhos/cm.

more eminent is the improvement of dissipation factor. 40

This fact is quite advantageous in continuous commercial operation.

Insulating paper A and insulating paper B were respectively subjected to extraction treatment by employing the TABLE 6 Current 100 0. tan 5 Applied under of treated Conductivity Rate of deionized water supplied voltage stationary er of discharged into vessel (liter/min.) (v.) state (a.) (percent) water (Ills/Om.)

Insulating paper A untreated 177-0. 180

TABLE 7 Current 100 C. tan 6 Applied under 0! treated Conductivity Rate oideionizedwatersupplied voltage statlon y pap r of discharged into vessel (liter/min.) (v) S a e (P r water (no/cm.)

EXAMPLE 8 sameapparatus in Example 1 except that the material of Kraft pulp obtained by digesting wood chips was sufliciently washed with industrial water. The resultant pulp was .then beaten in industrial water and made into cable insulating paper by a Fourdrinier paper machine using industrial water. The cable insulating paper thus produced (hereinafter referred to as insulating paper B) had the following properties:

the anode was changed. The treating conditions were: paper feeding rate, 120 cmJmin. (treating time: 10 sec.); conductivity of distilled water as liquid medium, 2 micro mhos/cm.; supply rate of the water, 1 liter/min.; and DC. voltage applied, 500 v. As shown in Table 9, platinum or carbon rod (graphite) was used as the anode. The treated paper was sufiiciently washed with deionized water having a conductivity of 1 micro mho/cm. and was Thickness 0-105 then dried. The characteristics of the treated papers are pp f denslty listed in Table 9 together with the untreated insulating {eslslance (Gurley -4 576 paper A and B for comparison. The extraction treatment Tellslltrength of the present invention resulted in hardly any variations Eloflgatlonipel'cem) in apparent density, air resistance, tensile strength and The same apparatus asin Example 1 in which the electrodes were spaced apart by 1.0 cm. was used. Extraction treatment was conducted in the same manner as in Exbreakdown voltage except in dissipation factor. Of these characteristics, the breakdown voltage was measured in accordance with the method described in P. Gazzana 17 Priaroggia and G. Palandri, Research on the Electric Breakdown of -Fully Impregnated Paper Insulation for High voltage Cables, AIEE Trans. (Power Apparatus and Systems), vol. 74, pp. 1344-1345 (1956), except that dodecyl benzene was employed for impregnation instead of conventional mineral O.F. cable oil. The dissipation factor measured at various temperatures was listed in Table 9. The other values of characteristics are determined in accordance with the method specified in I IS anode and cathode respectively in facing relationship with a distance of 1.3 cm. Insulating paper A 10 cm. in width was fed at a speed of 30 cm./min. (treating time, 40 sec.) between the electrodes fixed in position in a still liquid having a conductivity of 32 micro mhos/cm., while A.C. voltage of 200 v., 60 HZ. was applied. The paper treated Was then sufficiently washed with deionized water whose conductivity was 1.7 micro mhos/cm. and dried thereafter. The dissipation factor at 100 C. of the portion of C-2111. 10 paper which was taken out of the vessel 5 minutes after TABLE 9 After treatment In case of In case Before treatment platinum anode carbon anode Insulat- Insulat- Insulat- Insulat- Insulat- Insulatlng ing ing ing ing ing paper A paper B paper A paper B paper A paper B Thickness (mm) 0.100 0.105 0. 10 0.104 0.100 0.105 Apparent density (g./cm. 0. 67 0. 77 0.67 0.77 0.67 0.77 Arr resistance (Gurley sec./10O cc.) 2230 576 2280 580 2250 585 6. 41 11. 2 6. 49 11. 8 6. 48 11. 9 2.52 2.47 2.50 2.43 2. 49 2. 41 81 79 82 81 82 81 0. 26 0. 38 0. 13 0.22 0.11 0.22

Tan (percent) EXAMPLE 10 the extraction was found to have 0.150%. A portion of The extraction treatment was performed by employing the same apparatus as in Example 1. However, liquid medium inlet 12 and outlet 13 were closed and the vessel 1 was filled with about 6 liters of deionized water having a conductivity of 1 micro mho/cm. Insulating paper A 10 cm. in width was fed into the vessel 1 at a speed of 4 cm./min. (treating time: 5 min.), and passed through between a pair of the facing electrodes which was immersed in the deionized water. A D.C. voltage of 300 v. was applied to the electrodes. The conductivity of the deionized water at the center of the electrode beneath the paper reached 12 micro mhos/cm. in 2 minutes, 18 micro mhos/cm. in 5 minutes and 20 micro mhos/cm. in 10 minutes after starting the extraction treatment. The paper taken out of the vessel was fully washed with deionized water having a conductivity of 2 micro mhos/ cm. and then dried. The dissipation factor at 100 C. of the portion of the paper which was taken out of the vessel 10 minutes after the start of the extraction was found to have been improved to 0.160%.

EXAMPLE 11 The same apparatus as in Example 1 was employed. However, the extraction medium was supplied through the duct 13 positioned at the paper feeding side and discharged from the duct 12 located at the paper outlet side, the liquid extraction medium being flowed in the same direction as that of the paper. Insulating paper A was fed between the pair of the facing electrodes at a speed of 60 cm./min. (treating time: 20 sec.), while deionized water having a conductivity of 1 micro mho/cm. was supplied in the manner described above at a rate of l liter/min. DC. voltage of 300 v. was applied between the electrodes. The paper thus treated was sufiiciently washed with deionized water having a conductivity of 0.5 micro mho/cm. and was then dried. The dissipation factor at 100 C. of the paper was 0.140%. In stationary state, the conductivity of the liquid extraction medium just below the insulating paper A in the center portion of the electrode was 40 micro mhos/cm.

EXAMPLE 12 The same apparatus as in Example 10 was employed except that a pair of carbon electrodes was employed as the treated paper adjacent to the above paper portion was cut out as a specimen and was impregnated with conventional mineral O. F. cable oil whose dissipation factor at C. was 0.05%. Dissipation factor of this paper was found to be 0.152%.

EXAMPLE 13 In this example, extraction treatment was carried out by using the vessel shown in FIG. 4. In the vessel 1, 210 cm. in length, 40 cm. in width, 10 cm. in depth, mercury was filled to the depth of 0.8 cm. as a cathode. Three platinum anodes, 50 cm. in length and 30 cm. in width respectively, were disposed apart from one another by 3 cm. The distance between the anode and the cathode was adjusted to 2 cm. The respective inlet ducts 45, 46, 47, 48 had diameter of 0.5 cm. and the outlet duct 49 had height of 3.5 cm.

Insulating paper A 30 cm. in width was fed between the electrodes at a speed of 45 m./min. (treating time: 2 sec.), and water having a conductivity of 8 micro mhos/ cm. was supplied to the space between the electrodes through the inlets 45, 46 at a flow rate of 15 liters/min. in total in the direction opposite to the moving direction of the paper and overfiowed from the outlet 49 to keep the level of liquid medium constant. DC. voltage of 300 v. was applied to the electrodes. In this example, inlets 47 and 48 were kept closed. In a few minutes the stationary state in conductivity distribution was established. In the stationary state the conductivity of water just below the paper at the end of the anode on the paper inlet side and that of the discharged water were both micro mhos/cm. The paper was then sufiiciently washed with deionized water having a conductivity of 0.5 micro mho/cm. and then dried. The dissipation factor at 100 C. of the treated paper was 0.136%, showing that a distinguished reduction of the dissipation factor was achieved as compared with 0.180% of the untreated paper.

EXAMPLE 14 The chemicals given in Table 10 were added respectively in a proportion of 6 to 50 ppm. to deionized water having a conductivity of 0.5 micro mho/cm. to prepare three kinds of liquid extraction medium having a conductivity of 30 micro mhos/cm. This liquid extraction medium was then supplied at two different rates given in Table 10 into the same vessel 1 as in Example 13 through the inlet 48 so as to flow the liquid medium in the direction toward the paper feeding side. Through the inlets 45 and 46 the deionized Water having a conductivity of 0.5 micro mho/cm. was supplied at a rate of 5 liters/ min. in total to cause the water to flow toward the paper inlet side. The inlet 47 was kept closed. DC. voltage of 200 v. was applied to the electrodes. Insulating paper A was fed between the electrodes at a speed of 14 m./sec. The paper thus treated was sufliciently washed with the deionized water having a conductivity of 0.5 micro mho/ cm. and dried thereafter. The dissipation factors at 100 C. of these papers are summarized in Table 10. The conductivity of liquid extraction medium just below the paper atthe end of the anode on the paper inlet side is also shown in Table 10.

12 N HCl was added in a proportion of about 6 p.p.m. to. the deionized water having aiconductivity of 0.5 micro mho/cm. to prepare aliquid extraction medium whose conductivity was 30 micro mhos/cm. which was then supplied into the vessel as described in Example 13 through the inlet47 at a rate of 8 liters/min. so as to flow the liquid in the direction toward the paper feeding side. Further, under the same conditions as in Example 14, the deionized water having a conductivity of 0.5 micro mho/ cm. was supplied through the inlets 45, 46. The inlet 48 was kept closed. D.C. voltage of 200 v. was applied to the electrodes. In the same manner as in Example 14, insulating paper A 30 cm. in width was passed through between the electrodes at a speed of 14 m./min. The paper taken out of the vessel was sufllciently washed with deionized water having a conductivity of 0.5 micro mho/ cm. and was then dried. The dissipation factor at 100 C. of the treated paper was 0.140%, while in stationary state the conductivity of the liquid extraction mediumv just below the paper at the end of the anode on the paper feeding side was 119 micro mhos/cm.

EXAMPLE 16 The extraction treatment 'was carried out under the same conditions as in Example 15 except that the liquid extraction medium having a conductivity of 40 micro mhos/cm. prepared by adding HCl to deionized water having a conductivity of 0.5 micro mho/ cm. was supplied into the vessel through the medium inlet 47 and that the liquid extraction medium having a conductivity of 15 micro mhos/cm. prepared by adding Ca(OH) in a proportion of several p.p.m. to deionized water having a conductivity of 0.5 micro mho/cm. was also supplied into the vessel through the inlet 46 at a rate of 2 liters/ min. and that the deionized water having a conductivity of 0.5 micro mho/cm. was also supplied into the vessel through the inlet 45 at a rate of 2 liters/min. The duct 48 was kept closed. The dissipation factor at 100 C. of the treated paper was 0.138%. The conductivity of the discharged liquid in stationarystate was 160 micro mhos/cm.

EXAMPLE 17 Extraction treatment was carried out under the same conditions as in Example 4 except that industrial water having a conductivity of 200 micro mhos/cm. was used as a liquid extraction medium. The paper was then suificiently washed with deionized water having a conductivity of 5 micro mhos/cm., the paper thereafter being dried. The dissipation factor at C. of the treated paper was found to be 0.145%. The result was exactly identical with that obtained in Example 4.

The conductivity of the industrial water was measured during treatment. In this example, although the supplied liquid had a conductivity of 200 micro mhos/cm., the conductivity of the discharged liquid had a reduced value of micro mhos/cm. in stationary state. The lowering of conductivity as seen in this example sometimes takes place depending upon the feeding speed and kind of paper, supply rate and kind of liquid medium, and kind of electrode material.

What we claim is 1. A method for producing electrically insulating paper having a reduced dielectric dissipation factor which comprises continuously passing paper through a liquid medium having a conductivity of at least 10 micro mhos/ cm. and between at least one pair of facing electrodes applied with voltage to thereby remove ionic substances from the paper and drying the paper thus treated.

2. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 1 in which said liquid medium is a water having a conductivity of at least 10 micro mhos/cm.

3. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 1 in which said liquid medium is an aqueous solution containing at least one of water-soluble ionic substances dissolved therein and having a conductivity of at least 10 micro mhos/cm.

4. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 3 in which said ionic substance is a watersoluble salt of inorganic acid.

5. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 4 in which said salt of inorganic acid is at least one member of the group consisting of NH CI, (NH SO NH4NO3, NaCl, Na SO NaNO Na CO NaHCO K2804, KNOg, CaCl Ca(N03)2, MgCl MgSO Mg(NO BaCI and Ba(NO 6. The. method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 3 in which said ionic substance is an inorganic acid, organic acid or alkali.

7. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 1 in which said liquid medium is an aqueous solution containing a water-soluble nonionic organic, substance and having a conductivity of at least 10 micro mhos/ cm.

8. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 7 in which said aqueous solution further contains an ionic substance.

9. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 1 in which said liquid medium has a. conductivity of at least 20 micro mhos/ cm. i

10. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 9 in which said liquid medium has a conductivity of 20 to 300 micro mhos/cm.

11. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 9 in which said liquid medium has a conductivity of 30 to 250 micro mhos/cm.

12. The method for producing electrically insulating paper having a reduced dielectric dissipation factor ac.- cording to claim 1 in which said paper to be treated is a high voltage insulating paper having an apparent density 21 of 0.5 to 1.3 g./cm. and an air resistance of at least 150 Gurley sec./ 100 cc.

13. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 12 in which said paper to be treated is an extra high voltage insulating paper having an apparent density of 0.55 to 0.95 g./cm. and an air resistance of at least 200 to 5000 Gurley sec./ 100 cc.

14. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 1 in which said paper is continuously passed through the liquid medium at the speed of 0.1 to 800 m./min.

-15. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 14 in which said speed of the paper is in the range of 1 to 500 m./min.

16. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 15 in which said speed of the paper is in the range of to 300 m./min.

17. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 1 in which the voltage applied to said electrodes is in the range of 1 to 800 v. per cm. of the distance between the electrodes.

18. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 17 in which said voltage applied to said electrodes is in the range of to 500 v. per cm. of the distance between the electrodes.

19. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 1 in which the paper from which ionic substances are removed by extraction is washed with a washing water having a conductivity of less than 20 micro mhos/cm.

20. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 19 in which said washing water has a conductivity of not more than 6 micro mhos/cm.

21. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 1 in which said liquid medium is flowed in a definite direction at a constant rate.

22. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 21 in which said liquid medium is flowed toward the counter direction to the moving direction of the paper.

23. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 21 in which said liquid medium is flowed toward the same direction as the moving direction of the paper.

24. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 21 wherein said constant flow rate of the liquid medium is in the range of 0.05 to 500 m./min.

25. The method for producing electrically insulating paper having a reduced dielectric dissipation factor according to claim 24 in which said constant flow rate of the liquid medium is in the range of 0.5 to 200 m./min.

References Cited UNITED STATES PATENTS 2,092,489 9/1937 Williams 204l32 1,228,988 6/1917 Tate 204-132 1,226,279 5/1917 Tifiany 204l32 HOWARD S. WILLIAMS, Primary Examiner R. L. ANDREWS, Assistant Examiner mime STATES PATEN'E @FEFICE I CERTIFIATE @F QQRHEQTEUN Patent No. 3 I 720 Dated Octobe 1972 Invent0r(s) Hideo Fujita et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading insert Claims priority of Japanese Application Serial I No. 88149/68, filed December 2 1968.

Signed and sealed this 26th da of Jane 1973.

(SEAL) Attest:

EDWARDIP/LFLEYTCHERJR. ROBERT GOTTSCHALK Attesting Officer I a Commissioner of Patents FORM PC4050 (10-69) 'USCOMM-DC 60376-P59 mcz 195s o-nna-sa-a