US3705847A - Method for forming a uniform continuous web of paper - Google Patents

Abstract

A METHOD AND APPARATUS ARE DISCLOSED FOR FORMING A CONTINUOUS WEB OF PAPER UTILIZING THE PRINCIPLES OF ELECTROPHORESIS AND ELECTRO-OSMOSIS. A UNIFORM AQUEOUS DISPERSION OF PAPER-MAKING FIBERS IS FLOWED BETWEEN A PAIR OF HORIZONTAL, SPACED-APART, ELECTRICALLY CHARGED ENDLESS BELTS WHICH SERVE AS ELECTRODES FORMING A DEPOSITION ZONE. THE PAPER-MAKING FIBERS ARE ATTACHED TO ONE ELECTRODE BY ELECTROPHORESIS AND WATER TO THE OTHER ELECTRODE BY ELECTROOSMOSIS. A ELECTROMECHANICAL VIBRATOR VIBRATES THE TROUGH JUST PRIOR TO ENTRY OF THE AQUEOUS DISPERSION BETWEEN THE ELECTRODES TO MAINTAIN THE FIBERS IN SUSPENSION AND UNIFORMLY DISPERSE THEM WITHOUT FLOCCULATION. ELECTRICALLY NONCONDUCTING GUIDE BLOCKS ARE MOUNTED IN CONTACT WITH THE EDGES OF THE ENDLESS BELTS IN THE AREA OF

THE ELECTROSTATIC FIELD TO MAINTAIN THE PROPER ELECTRODE SPACING. AFTER THE ELECTRODE AND FIBER MAT ARE REMOVED FROM THE DEPOSITION ZONE, THE FIBER MAT IS SEPARATED FROM THE ELECTRODE SURFACE BY AN ADJACENT ELECTRICALLY CHARGED SURFACE WHICH TRANSFERS THE MAT. RESIDUAL WATER IN THE MAT IS REMOVED SIMULTANEOUSLY WITH THE TRANSFER BY ELECTROOSMOSIS. THE CELLULOSE FIBER MATS, PARTICULARLY USEFUL AS CAPACITOR TISSUE, HAVE BETTER ELECTRICAL PROPERTIES THAN CAN BE OBTAINED ON CONVENTIONAL PAPER-MAKING MACHINES.

Description

L- W- STILES Dec. 12, 1972 METHOD FOR FORMING A UNIFORM CONTINUOUS WEB OF PAPER Filed Sept. 21. 1970 //Vl/ENT0R1 LESTEFM 577155 15 United States l atent O 3,705,847 METHOD FOR FOG A UNIFORM CONTINUOUS WEB OF PAPER Lester W. Stiles, Longview, Wash, assignor to Weyerhaeuser Company, Tacoma, Wash. Filed Sept. 21, 1970, Ser. No. 73,779 Int. Cl. B011; /02

U.S. Cl. 204181 4 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus are disclosed for forming a continuous web of paper utilizing the principles of electro phoresis and electro-osmosis. A uniform aqueous dispersion of paper-making fibers is flowed between a pair of horizontal, spaced-apart, electrically charged endless belts which serve as electrodes forming a deposition zone. The paper-making fibers are attracted to one electrode by electrophoresis and water to the other electrode by electroosmosis. An electromechanical vibrator vibrates the trough just prior to entry of the aqueous dispersion between the electrodes to maintain the fibers in suspension and uniformly disperse them without flocculation. Electrically nonconducting guide blocks are mounted in contact with the edges of the endless belts in the area of the electrostatic field to maintain the proper electrode spacing. After the electrode and fiber mat are removed from the deposition zone, the fiber mat is separated from the electrode surface by an adjacent electrically charged surface which transfers the mat. Residual water in the mat is removed simultaneously with the transfer by electroosmosis. The cellulose fiber mats, particularly useful as capacitor tissue, have better electrical properties than can be obtained on conventional paper-making machines.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to a method and apparatus for forming continuous webs of paper through use of electrophoretic and electro-osmotic phenomenon from an aqueous dispersion of papermaking fibers.

(2) Prior art relating to the invention Conventional paper-making machines form a paper mat by retaining the paper-making fibers on a wire mesh, e.g., Fourdrinier wire. The water in the aqueous dispersion of paper-making fibers is forced through the wire by gravity, vacuum, surface tension, or mechanical pressing. Fine fibers and other particles are removed with the water by these methods. In the apparatus and method of this invention no Fourdrinier wire is used. In its place are a pair of spaced-apart, oppositely charged conducting surfaces serving as electrodes.

Electrodeposition by electrophoresis is well known and is used for industrial painting and for forming insulating coatings on metals. For example, U.S. Pat. Nos. 892,188 and 894,070 describe methods and apparatus for extracting water or other liquids from such materials as peat pulp. Other known methods and apparatus are shown by the following patents to Huebner, U.S. Pat. Nos. 2,224,- 391; 2,615,822; and 2,680,079.

It was not heretofore known, however, that a continuous web of paper could be formed by electrophoresis, nor that a machine to form a continuous web of paper by electrophoresis could be devised. In the apparatus of the present invention only papermaking fibers are consumed. The Water is removed continuously from the aqueous dispersion of fibers and can be reused in processing more stock.

Patented Dec. 12, 1972 The method and apparatus of this invention are primarily useful in making thin capacitor tissue. Thin capacitor tissue made with the method and machine of this invention includes a higher percentage of fibers, col loidal particles and other material, such fine particles usually not retained when a Fourdrinier wire is used. In addition, a capacitor sheet can be made with .greater uniformity because the fibers and other particles in the stock are continuously being furnished to fill open pores and thin areas.

SUMMARY OF THE INVENTION A uniform aqueous dispersion of paper-making fibers is continuously flowed at a regulated rate between a pair of spaced-apart, continuously moving, oppositely charged endless belts which serve as conducting surfaces or electrodes and form a deposition zone. The fibers are generally attracted and migrate to the positive electrode or conducting surface and are deposited as a fiber mat on the surface of the endless belt. The water is attracted by electro-osmosis and gravity to the opposite electrode. The fiber mat formed on the positively charged conducting surface is removed from the conducting surface and transferred to another surface by positively charging an adjacent moving surface in contact with the mat. Additional water in the mat is also removed during the transfer by electro-osmosis.

A vibrating trough, mounted on the machine just prior to entrance between the two conducting surfaces, maintains the fibers in suspension and uniformly dispersed without flocculation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 represents a vertical transverse section of one form of the apparatus for forming paper webs; and

FIG. 2 is a section view along line 2-2 of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, the apparatus of this invention comprises stock chest 2 in which paper-making stock 1, consisting of pulp fibers and water, is stored. The stock at this point is made up to a predetermined consistency and beating degree. An agitator 3 is run at a speed which prevents settling but does not cause introduction of air. Submerged pump 4 transfers the stock to head box 6 through line 5. Excess stock is removed to the stock chest by line 11. The slice 9 is set at a level so that the stock level is just slightly above the upper electrode 13. The head adjustment 10 is set to provide a predetermined flow rate. An electromechanical vibrator 7 is attached to a section of trough 8 to maintain the fibers in suspension and uniformly disperse them without flocculation. Vibration of this section of the trough is isolated from the ends of the trough by rubber connection 8a and 8b. The vibrator used, for example, may be one operated on a sixty cycle line driving the blade at cycles per second. The aqueous dispersion of paper-making fibers is fed from head 'box 6 through trough 8 between a set of endless, flexible conducting belts which serve as electrodes forming a deposition zone 50. The rate of flow of paper-making fibers between the belts can be adjusted by adjustment of slice 9. The belts may be of stainless steel or other conducting material.

The electrodes are oppositely charged. Usually the upper electrode is positively charged and the lower electrode negatively charged. Since pulp fibers are normally negatively charged they are attracted to and migrate to the positive electrode and the water migrates to the lower or negative electrode. The electrodes can be made as wide as desired for the size of paper sheet wanted. The two conducting surfaces 12 and 13 are in continuous movement. The speed at which the conducting surfaces are run for adequate paper formation can be theoretically calculated based on moving a charged particle from the surface of the negative electrode to the surface of the positive electrode according to the classical Helmholtz equation:

where v is the average velocity of a particle E is the electrode voltage difference,

L is the electrode separation,

Z is the Zeta potential of the particles, D is the dielectric constant of water, and 1 is the viscosity of water.

The speed is proportional to the active electrode length as can be seen from the equation. Endless belts 18a or 18b of rubber or similar material form seals at the edges of the electrodes as can be best seen in FIG. 2. Other nonconducting materials than rubber can be used where suitable. Endless belts 18a and 18b are trained over idler pulleys 40a and 40b which guide the belt seals. Stationary nonconducting blocks 19a, 19b, 20a and 20b are mounted adjacent the edges of electrodes 12 and 13. The electrodes slide over the blocks which function to maintain the proper electrode gap between electrodes 12 and 13. It is essential that spacing of the electrodes be maintained constant in order to obtain a uniform web of paper.

As the aqeuous dispersion of paper-making fibers is fed between the electrodes 12 and 13 into deposition zone 50, the papermaking fibers migrate to the upper or positive electrode by electrophoresis and the water is attracted to the lower electrode by electro-osmosis and gravity. A fiber mat 23a is thus formed on the upper belt electrode 13. The water flows by gravity into tank 22. Doctor blade 26 and 27 can be placed as shown in the drawing to scrape the respective conducting belts clean and dry before deposition of additional fibers.

The fiber mat 23a clings to electrode surface 13 as it passes out of the area of electrostatic force in the deposition zone 50 between electrode surfaces 12 and 13. To remove the fiber mat from the surface of electrode 13 it is necessary to contact the fiber mat with a more positively charged surface. As shown in FIG. 1 a papermaking press felt 25, trained around electrically conducting rolls, driven and supported by rolls 26 and 27, picks up the mat from upper electrode surface 13. An idler roll 29 guides the paper-making felt tangent to drum 17 and the upper belt 13 to provide optimum pressing conditions. The spacing between roll 17 and 26 is adjusted for optimum pressing and pickup. Roll 26, being positive with respect to the upper electrode 13, transfers the fiber mat 23a to the paper-making felt by electrophoresis. Residual water in the mat is removed at the same time by electro-osmosis and flows to collection area 31 where it is pumped out by line 32. Roll 24 functions to form area 31 so as to catch the residual water.

Roll is driven in the direction shown and is positively charged with respect to roll 27 so that mat 23b is transferred to roll 30 by electrophoresis. Residual water is forced into the felt 25 by electro-osmosis and removed by a suction box or other suitable means 28. The fiber mat 23c is then drawn off roll 30 and is further pressed and dried by conventional means.

The electrode surfaces 12 and 13 and the pickup rolls 26, 27 and 30 of FIG. 1 are powered by a direct current power source. A suitable power circuit, for example, is an electrode power circuit capable of delivering about 500 volts negative potential with respect to a common 4 ground and a pickup power circuit capable of delivering about volts positive potential with respect to a common ground. Contacts 35 and 38 of FIG. 1 are connected to both power supplies. Contacts 37 and 39 are attached to positive output terminals while contact 36 is connected to a negative output terminal.

The apparatus of this invention is primarily suitable for making thin capacitor tissue of increased uniformity over that made using conventional paper-making machinery. Capacitor tissue produced by the apparatus of this invention is superior to capacitor tissue made on conventional paper-making machinery. Such paper has the intrinsic characteristic of greater inclusion of fines, colloidal and other particles which contribute to its uniformity, and consequently its electrical insulating properties.

EXAMPLE A sheet of thin capacitor tissue was made by passing an aqueous dispersion of paper-making fibers at a temperature of approximately F. and a consistency of approximately 0.5% by weight of pulp between a pair of horizontally spaced-apart electrode surfaces such as shown in FIG. 1. The active electrode length was 12" and the width 3". The spacing between the electrodes was maintained at A2" and the machine speed at approximately 10 feet per minute. An electrode potential difference of 50 volts was maintained between the electrodes 12 and 13 at a current flow of 2 amperes. Pickup rolls 26 and 30 were maintained at a potential dilference of volts from that of conducting electrode surface on which the paper-making fibers were deposited. During passage between the electrode surfaces the paper-making fibers migrated and were deposited in a uniform manner on the surface of the positively charged upper electrode surface. The water migrated toward the lower or negative electrode and subsequently ran into a storage tank. The water removed by electro-osmosis was examined and observed to be much cleaner than water from conventional paper-making machines, indicating that more of the fibers were being deposited in the formation of the fiber mat and not being lost.

The paper made by the above apparatus was found to have superior electrical formation as determined by a Mercury Electrode Tester as compared to conventional capacitor tissue made on standard paper-making machinery. Electrophoretic samples tested at 100 volts or more had 18% fewer counts per foot than standard capacitor tissue. Electrophoretic samples formed at 1 /2 amperes and tested at 100 volts compared with standard paper, had 55% fewer counts per foot. The above values show conclusively that the paper formed by the above electrophoretic method has superior electrical formation.

What is claimed is:

1. A method for continuously making paper webs from an aqueous dispersion of paper-making fibers which comprises:

(1) continuously flowing at a regulated flow rate a uniform aqueous dispersion of paper-making fibers between a spaced-apart set of continuously moving, oppositely charged conducting surfaces, the conducting surfaces creating between them an electrostatic field extending the length and transverse width there- I (2) maintaining the electrostatic field between the conducting surfaces at a predetermined potential such that the paper-making fibers in the dispersion migrate to and are deposited by electrophoresis on one of the conducting surfaces, forming a fiber mat of predetermined thickness,

(3) removing the fiber mat from the conducting surface and, at the same time, removing residual water from the paper mat by bringing the mat into contact with 5 6 a charged surface having a potential sufficient to tional forces sufficient to maintain the fibers in suspension attract the paper mat to said charged surface, and and uniformly dispersed without flocculation.

(4) drying and pressing the mat.

2. Method according to claim 1 wherein said set of e en C t d conducting surfaces are horizontal and substantially 5 UNITED STATES PATENTS parallel.

3. Method according to claim 1 wherein the upper fi ggfli i i conducting surface is positively charged and the lower 6/1969 Heron et a1 conducting surface is negatively charged, the paper-making fibers being attracted by electrophoresis to the upper 10 HOWARD S. WILLIAMS Primary Examiner electrode and the water being attracted to the lower conducting surface by electro-osmosis. s, 31, X

4. Method according to claim 1 including subjecting 204 300 the aqueous dispersion of paper-making fibers, prior to their entry between the conducting surfaces, to vibra- 15

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