US3676294A - Method and apparatus for feeding fibers to headbox in paper-making using an electrical field - Google Patents

Abstract

A METHOD PRODUCING NONWOVEN SHEET MATERIAL COMPRISING IN WHOLE OR IN PART STAPLE FIBERS BETWEEN ABOUT 1/4 TO 1 1/2 INCHES LONG WHICH INCLUDES THE STEPS OF FEEDING THE FIBERS DRY AT A PREDETERMINED RATE TO THE BREAST BOX OF A WET LAID FORMING MACHINE BY MEANS OF AN ELECTRICAL FIELD WHICH ACCELERATES THEM TOWARD THE SURFACE OF THE AQUEOUS MEDIUM IN THE BREAST BOX. THE ELECTRICAL FIELD IS PRODUCED EITHER BY THE DIRECT APPLICATION OF A VOLTAGE TO THE FIBERS OR BY SPONTANEOUSLY CHARGING THEM WITH STATIC ELECTRICITY. THE BREAST BOX ACCORDING TO THE INVENTION INCLUDES AN ELECTRODE WHICH MAINTAINS THE MEDIUM AT GROUND POTENTIAL. THE APPARATUS ALSO INCLUDES A FIBER BASKET WITH A PERFORATED BOTTOM ABOVE THE BREAST BOX THROUGH WHICH THE FIBERS ARE ISTRIBUTED INTO THE MEDIUM. THE BOTTOM OF THE BASKET COMPRISES AN ELECTRODE FOR CHARGING THE FIBERS. THE BREAST BOX ALSO INCLUDES A BAFFLE PLATE WHICH DIRECTS THE IMMERSED FIBERS DOWNWARDLY INTO THE REGION OF THE SHEET FORMING ZONE WHERE THEY ARE UNIFORMLY MIXED WITH THE MEDIUM BEFORE BEING LAID DOWN ON A FORMING WIRE.

Description

July 11, 1972 F. OPDERBECK ETAL 3,676,294

METHOD AND APPARATUS FOR FEEDING FIBERS To HEADBOX IN PAPER-MAKING USING AN ELECTRICAL FIELD Flled June 23, 1970 United States Patent Ofice US. Cl. 162-192 Claims ABSTRACT OF THE DISCLOSURE A method producing nonwoven sheet material comprising in whole or in part staple fibers between about to 1% inches long which includes the steps of feeding the fibers dry at a predetermined rate to the breast box of a wet laid forming machine by means of an electrical field which accelerates them toward the surface of the aqueous medium in the breast box. The electrical field is produced either by the direct application of a voltage to the fibers or by spontaneously charging them with static electricity. The breast box according to the invention includes an electrode which maintains the medium at ground potential. The apparatus also includes a fiber basket with a perforated bottom above the breast box through which the fibers are distributed into the medium. The bottom of the basket comprises an electrode for charging the fibers. The breast box also includes a bafile plate which directs the immersed fibers downwardly into the region of the sheet forming zone where they are uniformly mixed with the medium before being laid down on a forming wire.

BACKGROUND OF THE INVENTION This invention relates to a method and apparatus for the production of nonwoven sheet material, or nonknitted flat articles, and more particularly to a method and apparatus in which fibers are fed'to the breast box of a forming machine and are formed into a sheet on a foaming wire by a wet laid process.

It is known to make nonwoven sheet material, such as paper, by a wet laid process which includes dispersing the fibers in water by agitating them in a pulp chest and after possibly diluting the dispersion further or adding one or more auxiliary substances, or both, by feeding the resulting fiber suspension to the breast box of a papermaking (i.e., forming) machine where the sheet is formed on a forming wire and the water is drained through the wire. This conventional process is not entirely satisfactory and has the drawback that it must be carried out at relatively low fiber concentrations, i.e., at pulp densities or consistencies of no more than about 1-2%. The term consistency as used herein means the percent by weight of the solids in the water. How high the consistency can be raised depends essentially on the average length of the fibers in the suspension. However, even if, by careful dispersing techniques with or without the use of dispersing agents, it is possible to obtain a uniform consistency dispersion, this uniformity may be destroyed in the pipelines and pumps between the pulp chest and the breast box of the forming machine to which the suspension is fed. If the individual fibers agglomerate and become knotted or lumpy in this process, they produce defects in the finished sheet which may cause it to tear easily or to be rejected, or both.

Agglomeration of the fibers is particularly likely when longer length fibers are used, so much so in fact that 3,676,294 Patented July 11, 1972 with fibers more than 6 mm. long it is necessary to use extremely high dilutions to avoid this problem. Handling such dilutions, however, requires large amounts of water and larger equipment with its attendant increased power consumption and cost.

Nonwoven sheet materials which include longer length staple fibers are becoming more important because they give the sheet much greater strength. In addition, longer length staple fibers give the sheet material drape and hand characteristics more like woven and knitted fabrics than like paper. The term stable fibers as used herein, means lengths of natural fibers, synthetic fibers or semi-synthetic fibers (such as for example, rayon which comprises regenerated cellulose). The longer length stable fibers are preferably between about A to 1% inches long. Materials which include these fibers may be made by a wet laid process and produced so much faster and less expensively than knitted or woven fabrics that they can be thrown away after a single use. This disposable feature eleminates the expense and need for washing and dry cleaning.

Sheet material products of this type have become known under the name of nonwovens and have become popular recently particularly in the sanitary field where they are used as covers for diaper inserts and sanitary napkins, as hospital bed linens and as physicians smocks. In addition, they are used as covers for single use blankets in tourist hotels and camps, as curtain and as certain articles of clothing. In making all of these products, however, the problem of handling the longer staple fibers is present. On the one hand, the longer the fibers are the better are the properties of the material. 0n the other hand, the longer the fibers the greater the dilution of the dispersion required to handle the fibers without agglomeration and other problems. This pushes the cost of the material up and even at extreme dilutions does not permit adequate handling of fibers longer than about 12 mm.

There is, therefore, a great industrial need for a new process which makes it possible to be able to utilize the high production rate and other advantages of the wet laid forming process without an uneconomical and extremely high dilution of the fiber suspension.

It is, therefore, one object of this invention to provide an improved wet laid nonwoven sheet forming method and apparatus.

Another object is to provide an improved method and apparatus for adding fibers to an aqueous medium.

A further object is to reduce the amount of water required in a wet laid process and apparatus for forming nonwoven sheet material.

Still another object is to provide an improved wet laid nonwoven sheet material manufacturing process which handles long synthetic fibers at high concentrations without forming knots, lumps or other defects in the finished sheet.

Yet another object is to provide an improved apparatus for adding dry fibers to an aqueous medium.

A still further object is to provide apparatus having the above characteristics which circulates less water than prior known devices in forming nonwoven sheet material comprising longer staple fibers.

Further, other and additional objects and advantages will become obvious from the brief summary and detailed description of the invention taken in connection with the drawings and the appended claims.

SUMMARY OF THE INVENTION One aspect of a method according to the invention comprises feeding dry staple fibers by an electrical field directly into the open breast box of a forming machine containing an aqueous medium into which the fibers are advanced. The electric field causes the fibers to repel each other and accelerates them toward the aqueous medium. Although the theory of how the process works is not known in full detail, the advancing and repelling effect is probably due to the fact that the medium contains a certain quantity of salts and thus is electrically conductive. It acts as a ground electrode. Thus, upon immersion of the fibers into the surface of the medium the fibers become electrically discharged and begin to sink. They are then drawn by the movement of the medium toward the wire on which the sheet material is formed.

According to another aspect of the invention the electric field is produced either by the direct application of from between about 2000 to 5000 volts D.C. to the fibers, or by spontaneously charging the fibers to within this range by static electricity. The precise voltage to which the fibers are charged using either or both of these methods depends on the type and character of the fibers being used.

One aspect of the apparatus according to the invention includes providing a fiber basket above the breast box of the forming machine and having the bottom of the basket provided with openings to allow the fibers to be distributed into the water medium below. The basket and its bottom preferably form an electrode which helps electrically charge the fibers either through a generator, a static electricity producing brush, or both. A brass plate in the breast box itself preferably serves as the second electrode which maintains the aqueous medium at ground potential.

Another aspect of the apparatus according to the invention comprises providing a bafile plate in the breast box of the forming machine between a steeply inclined forming wire and the point at which the fibers are added to the medium. The purpose of the baffle plate is to mix the fibers in the medium sufiiciently to ensure uniform formation of the sheet. This is particularly important in those cases in which the medium comprises a wood pulp fiber dispersion. In such a case there is the danger that if the fibers are not uniformly mixed, the final product will be nonuniform and have more wood pulp fibers on one side and more longer staple fibers on the other. The baffle plate avoids this result by being placed and shaped so as to cause the medium and the sinking staple fibers to flow downwardly past the baifie plate in the region of the forming wire so that both types of fibers will be uniformly mixed as they are laid down on the wire.

One advantage of the method and apparatus according to the invention is that the concentration of longer length fibers in the breast box of the forming machine can be raised to higher levels than heretoform known with the result that much smaller quantities of water need be used. This means that the equipment can be made smaller and more economical. Indeed, if the sheet contains no chemical or mechanical wood pulp at all, the pulp chests can be eliminated entirely reducing the equipment required still further. Even if such pulp is used, the pulp chests required may be reduced in number.

Another advantage is that the fibers introduced into the breast box need not be diluted beyond the extent customary in the conventional paper-making processes. In fact, when nonwoven sheet material is made from staple fibers alone these fibers can be added to the breast box at a rate which results in a higher concentration of fibers than has been used heretofore in the paper-making art. This is possible because the water in the breast box is required only as a fluid for forming the nonwoven sheet material and not for dispersing the fibers.

The surprising fact that according to the method of the invention even when longer fibers are used very high fiber concentrations can be employed is probably due to the fact that the fibers added to the aqueous medium by the electrical field are dispersed in the medium in an ideal manner and do not pass through any pipelines or pumping stations which could disturb the dispersion before it is formed into a sheet. Ordinary short fibers such as chemical and mechanical wood pulp can also be dispersed in the aqueous medium with the longer fibers resulting in an even higher total fiber density than with longer fibers alone. This is done by dispersing the short fibers in the medium before it is added to the breast box of the forming machine, by adding the longer fibers by the electrical field and by using the bafile plate to deflect the partially dispersed longer fibers downwardly toward the forming wire to mix them uniformly with the short fibers before laying them on the forming wire.

The principle of electrostatic flocking is known from the dry flocking of substrates, but it is entirely new and unexpected that fibers fed into the breast box of a forming machine using the method of this invention will enable the heretofore unsolvable technical problem of achieving high fiber concentrations in a wet laid nonwoven process to be solved.

It has been found particularly advantageous to form the bottom of the staple fiber basket from a screen having between about 1-16 openings per inch of a size from between about 1-20 square mm. Preferably the screen is vibrated at a rate of between /2--10 cycles per second through a distance of between about 2-10 mm. to facilitate the emergence of the staple fibers through the screen at a uniform rate. Naturally, the mesh of the screen may vary with the small openings passing the short fibers and the bigger ones passing the long fibers.

The sheet material is formed on a moving forming wire in the forming machines sheet forming zone.

BRIEF DESCRIPTION OF THE DRAWINGS The figure is a schematic view in elevation of an apparatus for carrying out the method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION A preferred embodiment of the apparatus according to the invention includes a forming machine having a breast box 1 adjoining a steep endless forming wire 2 behind which are located a plurality of suction boxes 3. A brass plate 4 is mounted on the inside of the breast box as a ground electrode. The forming wire 2 moves upwardly through the breast box and is guided and tensioned by a plurality of rollers 12, 13, 14, 15 and 16. Additional suction boxes 3 are located in the horizontal portion of the forming wire 2 between the rollers 12 and 13 to extract water from the just formed sheet. Below the return path of the wire and therefore below the rollers 13, 14, 15 and 16 a water collecting pan 17 is mounted.

The aqueous medium 6 which may contain ordinary wood pulp or mechanical wood pulp passes into the breast box 1 via a feed pipe 5 in the direction indicated by the arrow. Also mounted in the breast box is a bafile plate 11 which extends downwardly into the medium 6 between the feed pipe 5 and the forming wire 2. Above the breast box 1, between the bafile plate 11 and the feed pipe 5, is a flock distributing device comprising a staple fiber basket 7, a bottom screen 9 and a distributing roll 18 which preferably comprises a brush for generating a spontaneous electrostatic charge in the fibers 8. These fibers are added to the basket 7 by means of a conveyor belt 19 which moves around a roller 20 located over the basket. The screen 9 which forms the bottom of the basket serves as an electrode for imparting an electrical charge to the fibers 8. The screen 9 is electrically connected with one side of an electric generator 21 to impart the charge to the fibers. In a similar manner the brass plate 4 on the inside of the breast box 1 is connected to the opposite or ground side of the generator 21.

Connection of the generator to the screen 9 and the brass plate 4 in this manner creates an electric field between these two elements. Because the aqueous medium 6 always contains small traces of salt and is thus electrically conductive, the field in fact exists between the screen 9 and the upper surface 10 of the aqueous medium 6. Means (not shown) are provided for vibrating the screen 9 to facilitate passage of the fibers through the screen, after which they are accelerated by the field. As soon as the fibers 8 hit the water they are discharged and begin to sink. Before they sink very far, however, movement of the medium flows the long fiber dispersion past the baflle downwardly into the region of the breast box close to the forming wire where they are uniformly dispersed by the movement of the medium before being applied to the forming wire 2 on which the nonwoven sheet material is formed.

The following examples further illustrate the invention which is not limited to the specific materials disclosed nor to the particular parameters chosen:

EXAMPLE I In order to produce a nonwoven sheet material comprising 30% wood pulp fibers and 70% synthetic staple fibers wood pulp fibers are added in a 1% consistency aqueous dispersion to the breast box of a forming machine, a plurality of polyamide 12 desitex fibers 12 mm. long are added dry at a predetermined rate to the dispersion which is then formed into a finished sheet. The term desitex is a well-known measure of the fineness of a fiber and represents the weight in grams of a length of the fiber 10,000 meters long. The weight of the finished nonwoven sheet material is 49 grams per square meter and the tear longth is 3000 meters with a longitudinaltransverse ratio of 1.25 and an elongation of 12%.

EXAMPLE II In order to produce a nonwoven sheet material of 100% synthetic fibers, clear water containing no additives is fed to the breast box of a forming machine, polyamide 6 desitex staple fibers are introduced into the water at a predetermined rate by a flock strewer and are then formed into a finished sheet. The weight of the finished nonwoven sheet material is 50 grams per square meter. The tear length is 4000 meters and the longitudinal-transverse ratio is 1.25.

The bonding of the sheet material is accomplished by spraying a latex binder on both sides of the sheet. A total of about grams per square meter of binder is applied. The elongation of this sheet is EXAMPLE III In order to produce a special paper such as is used for instance, for identification papers and similar purposes, and 65% by weight of which comprises wood pulp, 15% acrylic plastic binder material and 6 desitex polyamide fibers 8 mm. long, the wood pulp is introduced into the breast box in an aqueous dispersion at a predetermined consistency. Then the acrylic plastic binder material is added to the dispersion and deposited on the wood pulp. Finally, the polyamide fibers are introduced into the dispersion at a predetermined rate by a flock strewer and the binder coated wood fibers and polyamide fibers are formed into a finished sheet. This finished sheet weighs 36 grams per square meter. The tear length is 1000 meters and the longitudinal transverse ratio is 1.3; elongation is 3%.

EXAMPLE IV For the production of a two-sided paper which comprises 55% by weight of rayon staple fiber an additional 15% by weight of acrylic resin binder and the final 30% by weight of 1.5 desitex wood pulp fluff 15 mm. long, a dispersion containing the rayon fiber at a predetermined consistency is fed to the breast box. Then the binder is added to the staple fiber suspension and deposited on the rayon. Finally, the wood pulp fiutf is introduced into the dispersion at a predetermined rate by means of a flock strewer. Then the fibers are formed into a finished sheet with the binder coated rayon fibers comprising one side of the sheet and the Wood pulp fluff comprising the other. The weight of this two-sided paper is 65 grams per square 6 meter. The tear length is 3000 meters, the longitudinaltransverse ratio is 1.3 and the elongation is 18%.

While the invention has been described in connection with a certain preferred process and embodiment which have been set forth in some detail, the invention is not in tended to be limited to this preferred method and embodiment. For example, though the invention has been described in connection with staple fibers which are of particularly advantageous use in the method, the method is not limited to this type of fiber. Likewise, though the flock distributing roll 18 has been described as a brush for gen erating an electrostatic charge on the fibers 8, it should be clear that it rotates and also serves to distribute the fibers over the screen so they fall through the screen at a predetermined rate. The ends of the brush contact the screen as the brush rotates. Various modifications and changes in the details of the embodiments and steps may be made within the spirit and scope of the invention.

What is claimed is:

1. A method of producing nonwoven sheet material comprising the steps of: forming a dry mixture of staple fibers exceeding 6 mm. in length, generating an electric field between the fibers and an aqueous medium such as a water or water-fiber stock in the breast box of a papermaking machine, applying the electric field to the fibers to accelerate them at a predetermined rate into the medium in the breast box, mixing the fibers in the medium by movement of the medium without external agitation to form a slurry of predetermined consistency and thereafter laying the fibers down on a forming wire through which the medium passes.

2. A method according to claim 1 wherein the electric field is produced by applying a voltage to the fibers.

3. A method according to claim 1 wherein the electric field is produced by generating an electrostatic charge on the fibers.

4. A method according to claim 1 wherein the electric field is produced in part by applying a voltage to the fibers and in part by generating an electrostatic charge on the fibers.

5. Apparatus for producing nonwoven sheet material from a plurality of staple fibers which comprises a breast box for receiving a water or water-fiber stock medium into which the fibers are dispersed, a forming wire onto which the fibers are laid to form the sheet and means for distributing the fibers dry into the medium in the breast box including means for creating an electric field between the distributing means and the medium in the breast box for accelerating the distributed fibers toward the medium.

6. Apparatus according to claim 5 wherein the distributing means includes a basket for holding a supply of fibers and the bottom of the basket is perforated to distribute the fibers through the perforations, and the bottom of the basket also comprises an electrode forming part of the electric field creating means, said means also including a ground electrode member mounted within the breast box and electrically connected to the medium.

7. Apparatus according to claim 6 wherein the breast box includes a sheet forming zone and the forming wire is inclined steeply upwardly in said zone, the breast box also including a bafile plate for deflecting the immersed fibers downwardly into the sheet forming zone.

8. A method of producing nonwoven sheet material comprising the steps of forming a dry mixture of staple fibers exceeding 6 mm. in length, adding an aqueous medium to the breast box of a sheet forming machine, generating an electric field between the fibers and the medium, accelerating the fibers at a predetermined rate by means of the field into the medium, mixing the fibers so accelerated in the medium by the movement of the medium in the box without agitation to a desired consistency and forming the resulting stock into a sheet.

9. A method according to claim 8 wherein the step of adding an aqueous medium comprises adding an aqueous dispersion of wood pulp at a predetermined consistency 7 8 to the breast box of the sheet forming machine and the 3,329,125 7/1967 Grossteinbeck et a1. 118-638 step of accelerating the synthetic staple fibers into the 3,3 41 19 Wirth 1 medium comprises accelerating the fibers into the wood 2 890149 6/1959 Muller 162 350 X pulp dispersion.

10. A method according to claim 9 wherein the step 5 3,228,825 1/1966 Waggoner X of adding the dispersion to the breast box comprises add- FOREIGN PATENTS ing a 1% consistency aqueous dispersion of wood pulp 452 595 11/1948 Canada.

fibers and the step of accelerating the staple fibers into the dispersion comprises accelerating 12 desitex 12 mm. LEON BASHORE, Primary Examiner long synthetic staple fibers in a dry condition into the 10 dispersion F. FREIFF, Assistant Examiner References Cited UNITED STATES PATENTS 1 1 2,742,185 4/1956 Landry 117 17X 17 7, 934 R, 118 636, 638, 162 327, 350, 380

3,275,496 9/1966 Sponsel 162-157 x

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